If you're going to be one of the expected million people hoping to glimpse Pope John Paul II during his historic visit here next weekend, you're probably wondering what your chances will be in that unprecedented crush of humanity.

What are the best vantage points? Where can you park? What streets will be closed? Will there be shuttle bus service? Where can you find toilets? What about emergency medical facilities? Should you bring your ailing grandmother to the Mall for the papal mass?

Government, private and church planners are feverishly completing preparations to meet those questions and other contingencies. But, they warn, no amount of advance planning will take care of everything in the face of the immense throng that is expected.

The systems of the city are simply too small. Streets, buses, subway trains, food outlets, medical emergency facilities, yes, even restrooms, will all be taxed far beyond ordinary capacity, planners say.

The planners urge patience and forbearance. "Bring a chair. Bring a book. Bring a Bible," said Metro communications director Cody Pfanstiehl.

"This will be the acid test for all of us," said William H. Rumsey, head of the Mayor's Special Events Task Force, a group of city service agencies responsible for coordinating logistical assistance at political demonstrations and other large public gatherings that have become a part of Washington life.

It will be a great "faith experience," said the Rev. Maurice T. Fox, communications director for the Archdiocese of Washington.

For Starters, keep in mind that the pope will be here for two full days. He will make several public appearances throughout the city and participate in a number of parades along major city streets. There will be ample opportunity to see him at occasions other than the most highly publicized one -- the huge outdoor papal mass on the Mall on Sunday, the last public event of his tour.

The first opportunity to see the pope will be next Saturday morning when he arrives about 11 o'clock by helicopter from Andrews Air Force Base at the Rainbow Pool just east of the Reflecting Pool near the Washington Monument. There, Mayor Marion Barry will present the pontiff the keys to the city. The area will be heavily barricaded and teeming with an official welcoming group of thousands of Catholic schoolchildren. Still, an enterprising onlooker who gets there early may be rewarded with a glimpse or two.

After that, you might want to position yourself along one of three parade routes the pope will take through downtown Washington during the day.

These include his trip up 17th Street and K Street NW to St. Matthew's Cathedral on Rhode Island Avenue NW for a special mass after he lands near the Reflecting Pool.

Then after lunch at St. Matthew's Rectory, he will travel down Connecticut Avenue, 17th Street and Pennsylvania Avenue to the White House around 1:30 p.m. for a meeting with President Carter.

Your last chance on Saturday will be around 4:30 p.m., when the pontiff goes from the White House down 17th Street NW to the Pan American Union building at Constitution Avenue NW for a reception by the Organization of American States.

Church officials cautioned that these parade times are approximate and subject to change and delays. Yesterday in Ireland, the pope was running more than two hours behind schedule by the end of the day.

An especially good vantage point to see the pope may be in front of St. Matthew's Cathedral and rectory in the 1700 block of Rhode Island Avenue NW. Officials say that while the pope is dining in the rectory between 12:45 and 1:15 p.m., he may appear or even speak briefly from a second floor window of the rectory.

The pope will probably not be visible to onlookers when he is at the White House and Pan American Union.

On Sunday, the pope will appear at Catholic University and the National Shrine of the Immaculate Conception in the morning.

At about 8 o'clock, he will greet univeristy students from the steps of the shrine facing Michigan Avenue, then enter the building for a prayer service and address to about 4,000 nuns. This event is not open to the general public.

However, when he leaves the shrine at about 9:15, he will travel in an open car for a short distance along Michigan Avenue to the Catholic University field house for a meeting with theologians and other academics (again, a close event).

An hour later, he will travel once more down a short stretch of Michigan Avenue to Trinity College for an ecumenical prayer service with representatives of several non-Catholic Christian churches.

The last chance to see the pope at Trinity will be at about 11 a.m., when he will offer specially arranged greetings for several hundred handicapped persons assembled on the campus lawn. The grounds will be generally closed to the public, but the pontiff may be visible from adjacent streets.

The pope is then whisked by a "closed" nonparade motorcade across town to the residence of the apostolic delegation, 3339 Massachusetts Ave. NW, for lunch and rest before going to the last and biggest event of his visit here -- the papal mass on the Mall.

Leaving the apostolic delegation at 2:30 p.m., he will come by closed motorcade toward the Mall area. At 17th Street and Constitution Avenue NW, he will slow to a "parade configuration," as police call it, waving to the crowds from his open limousine, as he goes more than a mile east along Constitution Avenue and then south on Third Street and finally west on Jefferson Drive to the Smithsonian Castle on the south side of the Mall. There he will "vest" for the mass, which is scheduled to begin at 3 p.m.

Standing atop a three-tiered stage especially constructed for the mass, the pope should be visible to most people standing in the 125-acre Mall area from Third to 14th streets, even though he may be only a speck in the distance. An elaborate sound system with scores of speakers scattered along the Mall should assure that his words will be heard by all. He will speak in English.

After the two-hour mass, the pope's motorcade will leave the Smithsonian Castle by going west on Jefferson Drive to 14th Street, then speed away to the apostolic delegation in preparation for his departure from the United States.

Some time after 6 p.m., he will cross Massachusetts Avenue to the Naval Observatory where a helicopter will take him to Andrews Air Force Base. There will be a brief farewell ceremony led by Vice President Mondale, and the pope's plane will leave for Rome around 7:30 p.m.

The mass on the Mall clearly will be the biggest and most confusing event for onlookers. Planners are arranging complex logistical support facilities in the general area, including first aid stations, lost persons booths, food vending carts, water supplies, Medivac helicopter service for heart attack victims and other seriously injured persons, shuttle bus service in and out the area and at least 500 portable toilets.

In addition to urging patience and forbearance, planners said worshipers should plan for a long day, bring picnic food and blankets, avoid driving close to the Mall and walk in to the mass or use shuttle buses.

Getting to the Mall and getting away from it after the mass will be difficult. Streets will be clogged, buses and trains jammed. People will have to wait in lines.

"Probably the easiest way to get to the Mall is to rent a canoe and come down Rock Creek," said one wag at the Smithsonian Institution.

Here is a rundown of transportation, food, medical and sanitation measures being planned for Sunday. METRO TRAINS

The subway will operate from 6 a.m. to midnight -- later if necessary. Trains will run every six minutes on all lines. More frequent service than that is planned after the mass on the Mall, probably around 5:30 p.m.

Metro will not permit patrons to leave the Smithsonian station at the mid-Mall exit because of its closeness to the papal stage and altar.

Subway riders may use the other Smithsonian exit at 12th Street and Independence Avenue or any of the L'Enfant Plaza exits near 7th and D streets SW, but they'll have to take a long walk to get to the Mall. Police are planning to cordon off an area between Independence Avenue and Jefferson Drive from Fourth to 14th Street as a secure "no-go" zone. Subway patrons coming out of stations adjacent to the zone will have to walk to either Fourth or 14th to get around to the Mall to the north of Jefferson Drive.

Metro, therefore, recommends riders to use either the Federal Center SW or the Federal Triangle stations.

The Farecard system for paying train fares will not be used. Passengers will pay a flat 50-cent cash fare for all trips by depositing exact change in screened metal barrels at station entrances.

On Saturday, the subway will operate its usual Saturday hours, from 8 a.m. to midnight, with trains running every 10 minutes. If personnel are available, more frequent service will be provided at some points in the day.The same 50-cent cash fare arrangement as Sunday will be required. METRO BUSES

City buses will run on regular Sunday schedules but with 800 available instead of the usual 300. At least 200 more buses will be used as shuttles to bring in people from outlying parking lots who have come from out of town on chartered buses. Metro is still working on final details of the shuttle bus system.

Regular bus fares will be charged on regular route buses.

On Saturday, buses will operate on a regular Saturday schedule. Because of the pope's movements through town, Metro officials expect there will be times when buses -- like other vehicles on the street -- will be delayed.

Police will close streets temporarily around St. Matthew's Cathedral and Catholic University, for example, when the pope goes to those locations. Traffic will be rerouted, thus causing some delays and congestion. CHARTERED BUSES

Out-of-town chartered buses will be parked at one of four outlying areas -- the Pentagon parking lots, the grounds of East and West Potomac parks and Lots 1, 2 and 3 of RFK Stadium. Shuttle bus service will be operated between the buses and the Mall area for those buses parked at the Pentagon and East and West Potomac parks. Riders whose buses park at RFK Stadium will take the subway to the Mall area. Bus and train riders will pay 50 cents each way for rides. DRIVING

Most streets in the Mall area, including Independence and Constitution avenues and streets crossing the Mall, will be closed to traffic at 6 a.m. Sunday. However, police will attempt to keep open the Southeast-Southwest Freeway, 23rd Street NW and the three tunnels under the Mall on Third, Ninth and 12th streets.

Drivers entering Washington on the 14th Street Bridge will probably be shunted onto the Southwest Freeway on Sunday. Drivers using Memorial and Roosevelt bridges will be routed north on 23rd Street.

Police are committed to keeping shuttle bus routes open at the expense of automobile traffic. A major shuttle bus route from the Pentagon traverses Washington Boulevard in Arlington and then crosses Memorial Bridge. Police say they will close those two roadways on Sunday to automobiles if it is necessary to keep the shuttles running.

The Shirley Highway inbound express bus lanes will open at 11 p.m. Saturday for buses only. They will close at 1 p.m. Sunday, two hours, before the scheduled mass on the Mall.

After the mass, southbound 14th Street Bridge traffic will be routed onto the express bus lanes under the tentative plans. The regular traffic lanes of the bridge will be turned over to pedestrains wishing to hike back to Virginia. PARKING

There will be no parking in the Mall area on Saturday or Sunday.

Most downtown Washington parking garages will be open both days. Some will give preference to their regular contract customers. The lots will charge "established rates," according to the Washington Parking Association.

On-street parking will be permitted in most downtown areas. Street-by-street details are still being developed.

Fringe parking on Sunday will be provided free at the following locations, but spaces are limited:

In the District of Columbia -- South Capitol Street fringe lot at South Capitol and Firth Sterling Street SE, and Carter Barron at 16th and Kennedy streets NW. Also, about 12,000 parking spaces are available at RFK Stadium for $2 a day.

In Maryland -- Sears White Oak Shopping Center, New Hampshire Avenue and Lockwood Road; Korvettes, 11800 Rockville Pike, Rockville, and Woodward and Lothrop/Geico, Wisconsin and Western avenues, Chevy Chase.

In Virginia -- Springfield Industrial Park at Backlick Road and Pentagon North Parking.

Shuttle bus service will be provided from most of these locations with final details to be announced later this week. MEDICAL EMERGENCIES

Ten first aid stations will be located on and around the Mall, staffed by physicians and nurses from the D.C. Department of Human Resources, Red Cross volunteers and litter bearers provided by the Knights of Columbus. In addition, there will be 13 ambulances, two mobile intensive care units and a specially designed golf cart with coronary care equipment for getting through crowds. Four police helicopters will be prepositioned near the Mall to airlift seriously ill or injured persons to the Washington Hospital Center. Red Cross workers will also man four "lost persons" stations. FOOD

Several mobile and stationary food concessions will be open in the Mall area. Also, cafeterias in three nearby Smithsonian museums -- Natural History, Air and Space, History and Technology -- will be open until 9 p.m. on Saturday and Sunday. Food capacity is limited to about 35,000 meals at the cafeterias, and officials urged people to bring their own food. TOILETS

In addition to toilet facilities in the museums and other public buildings in the Mall area, archdiocesan planners contracted for about 500 portable toilets to be clustered at various points. Planners said, however, that because of "aesthetic" considerations, no portable toilets will be located around the papal stage and altar.