Washington's Metro subway will take its first step toward becoming more than a glorified commuter railroad tomorrow night when its operating hours are extended to midnight on weekdays.

Five days later, on Sept. 30, Metro will inaugurate Saturday service between 8 a.m. and midnight, thus bringing the 25 miles of subway up to full operating status for six days a week. No Sunday service is planned for at least another year.

It will be a big step forward for Metro and a big test at the same time. If riders stay away, the Metro Board has made clear. the service will be cut back to weekdays once again. It is going to cost area taxpayers an extra $2 million in the current fiscal year to subsidize the extended service, and that figure assumes Metro's ridership projections will hold up.

If, however, significant numbers of area residents begin to use the train to go shopping to keep theater and restaurant dates or to take their families to the Smithsonian museums on Saturdays, political support will build to keep the extended service operating.

Metro's financial planners are projecting that another 9,100 people a day will ride the subway between 8 p.m. and midnight, and that an average of about 60,000 people will use it on Saturdays.

Metro was averaging better than 200,000 subway riders a day on its 6 a.m.-to-8 p.m. service during the tourist-heavy summer months and has dropped back to an average of slightly more than 190,000 a day since Labor Day.

Trains will continue to run every 5 or 6 minutes on both the Red and Blue lines during weekday rush hours. They will run every 10 minutes on both lines until 10 p.m. weekdays, then every 15 minutes until midnight.

On Saturdays, trains will run every to minutes between 8 a.m. and 10 p.m. then will run every 15 minutes until midnight. Metro's nonrush-hour fare of 50 cents for any trip in any direction will apply all day Saturday.

The last train will leave Dupont Circle station at midnight, the Silver Spring station at 11:46 p.m., Stadium-Armory station at 11:49 p.m. and National Airport station at 11:47 p.m. All four last trains will reach Metro Center at 12-04 a.m. so that passengers can transfer. Signs at each station will list departure times for the last trains through the station. Wallet cards listing the same information are supposed to be available at station kiosks.

Metro will close the entrances to some stations at 10 p.m. weekdays and all day Saturday. Those are: The north entrance at Silver Spring.

The 4th and D streets NW entrance at Judiciary Square.

The 12th and F streets NW entrance at Metro Center.

The entrance to Farragut North on the west side of Connecticut Avenue at L Street NW.

The L'Enfant Plaza entrance into the Department of Transportation building courtyard.

In addition, the Arlington Cemetery station and the Mall entrance to the Smithsonian station will be closed after 10 p.m. daily.

Metro's parking lots will be in operation at the Takoma, Fort Totten and Rhode Island Avenue stations, and the standard $1 fee will be charged. Montgomery County provides parking in its garage near the Silver Spring station.

The D.C. government charges $1.25 for each of 1,200 spaces near the Stadium-Armory station; another 800 spaces are available for $1.25 at a private lot at Pentagon City Station.

In conjunction with the expanded subway service, Metro has realigned schedules on about 150 bus routes to eliminate some duplicate subway and bus service on week nights and Saturdays or to coordinate off-hour bus service with the new subway schedule. New timetables on affected bus routes are supposed to be in the hands of bus drivers and available at subway station information kiosks.

A full-time subway system was one of the promises Metro made to the metropolitan area when the 100-mile system was designed in the late 1960s. Since those years, when it was assumed that Metro would make money, financial realities have done more to determine transit service decisions here than demonstrated or expressed need.

Federal officials, who are being asked to contribute 80 percent of the $3.5 billion or so needed to complete building the system, have expressed concern that the investment will not be worth it if Metro is going to restrict itself to operating weekday schedules with emphasis on the rush hours.

There is also some suburban-inner city conflict here, with the District of Columbia somewhat more interested in providing comprehensive transit service than some of the suburbs.

Metro General manager Theodore C. Lutz lobbied hard with area politicians to win approval for the expanded service over the objections of several local budget anaylsts. Lutz says he firmly believes that Saturday service in particular will bring a "revolution" in the way people can move around the metropolitan area.

Francis B. Francois, a Prince George's County Council member and an influential member of the Metro Board, joined in the unanimous board vote approving the service, but cautioned area residents, "Use it or lose it."