The Vatican has worked out final plans for Pope John Paul II to visit Washington Oct. 6 and 7, when he will climax a week-long U.S. tour by celebrating a mass, including communion, for an expected 1 million people crowding the Mall from the Capitol to the Lincoln Memorial.

Sources directly involved in the planning of the pope's visit to the United States confirmed details of his six-city itinerary yesterday. The official Vatican announcement is expected shortly.

The pontiff will arrive in Boston Oct. 1, address the United Nations in New York Oct. 2, vist Philadelphia Oct. 3 and Des Moines, for a few hours, Oct. 4. He will spend the nights of the 4th and 5th in Chicago, then arrive in Washington at midday Oct. 6, according to Roman Catholic church sources.

Pope John Paul first will visit St. Matthew's Cathedral on Rhode Island Avenue NW for a meeting with local priests. Later in the afternoon, he will go to the White House to talk with President Carter. On the morning of Oct. 7, the pope will visit the National Shrine of the Immaculate Conception and Catholic University in Northeast Washington.

His last public appearance in this country, before flying back to Rome, will be the mass on the Mall. Church and police officials expect him to address the largest crowd ever assembled there from atop a stage erected near the Smithsonian castle.

Church officials are discussing the use of as many as 1,500 eucharistic ministers to deliver communion to the crowds. The churchmen also have been talking with city and Metro officials about transportation, security and sanitary logistics.

George Berklacy, spokesman for the U.S. Park Service, said yesterday that such tremendous numbers of people at a single assembly are unprecedented in the city's history. Not even the biggest civil rights and antiwar demonstrations equaled the expectations for Oct. 7, Berklacy said. Though a million people watched the Bicentennial fireworks he noted, they were spread out and were not trying to focus on a single man.

"This is unique," Berklacy said. "We average 750 to 1,000 demonstrations a year. But this promises to be the biggest ever 'demonstration' -- even that word doesn't sound right for this event . . . We try to think what other events we've had of this nature and we come up with none." Hundreds, possibly thousands, of people are expected to start gathering on the Mall days in advance of the pope's appearance, Berklacy said. Traffic problems, which would be enormous in any case, may be complicated by the fact that the Redskins will be playing a home game the day of the mass.

"We still scratch our heads as we look at the Mall and the surrounding parkland, then look at the various sites that parking might be available -- but you know it won't be," said Berklacy.

Such logistical problems have yet to be resolved in any of the cities now planning to receive the pope. All expect crowds of hundreds of thousands. Church officials said yesterday that the telex machines between the Vatican and the Bishops' Conference here have been operating constantly for days trying to sort out all of the final details.

Pope John Paul had been expected to arrive in Boston at 1 p.m. Oct. 1, for example. A motorcade had been planned through some of the poorest and some of the most Catholic parts of the city to the Cathedral of the Holy Cross, according to Boston church officials.

Then there was to be a mass for several hundred thousand people on the Boston Common before still another motorcade to Cardinal Humberto Medeiros' residence.

But the latest word has the pope arriving at Logan airport at 4:30 p.m., and Boston officials fear some of the planned activities may have to be canceled.

The pontiff will fly into New York Oct. 2, visit St. Patrick's Cathedral, then address the United Nations early in the afternoon.

But New York officials have not been able to find an open space large enough, or manageable enough to accommodate the expected crowds.

Accordingly, the pope will visit both Yankee Stadium the afternoon of Oct. 2 and Shea Stadium the morning of the 3rd as he leaves the city.

After visiting Philadelphia's Basilica of Saints Peter and Paul on Oct. 3, the pontiff is expected to celebrate mass for as many as a million people, probably along the Benjamin Franklin Parkway, church officials said.

On Oct. 4 Pope John Paul will head for the heart of the midwest. Arriving in Des Moines, he will visit the Living History Farms Foundation, a 600-acre park-museum on the outskirts of the city.

Des Moines officials said there has been discussion of the pope's visiting a small rural parish to contrast with the cathedrals he will see in every city he visits.

But with plans for him to stay in the city only four or five hours before leaving for Chicago, church officials said the logistical problems of such a venture into the countryside make this unlikely.

In Chicago, after visiting Holy Name Cathedral, Pope John Paul will meet with America's Roman Catholic hierarchy at the National Conference of Bishops.

Plans also are being made for the pope and the bishops to celebrate an outdoor mass, probably on Oct. 5, with the most likely location being Grant Park along the shores of Lake Michigan.

Precise details of where the pope will go and who he will see in Washington after he arrives here Oct. 6 -- beyond the visits to the cathedral, the White House, the shrine and the mall -- are still being worked out amid a deluge of special requests from around the area.

The Rev. Maurice Fox, spokesman for the Washington archdiocese, said recently that scores of individuals and groups have written or called the archdiocese suggesting that the pope visit them because they feel they are a typical American family, or lay a wreath on some particular monument, or else bless this or that.

"I think," said Fox, "the constraints on his time will make almost all of this impossible."