When Metro opens its subway for weeknight and Saturday service late in September, midnight will really strike at 11:46 p.m. in Silver Spring.

General Manager Theodore C. Lutz told the Metro board yesterday that the best combined subway and bus schedule his staff could arrange would have subway service from downtown outward until midnight.

However, trains from all four operating directions will be scheduled to meet at Metro Center at 12:04 a.m., where they will be held until everybody has made the necessary transfers.

For all trains to arrive at Metro Center at 12:04 means, however, that the last train from Silver Spring must leave there at 11:46; the last train from National Airport must leave at 11:47 p.m. and the last train from Stadium-Armory must leave at 11:49 p.m. The last train from Dupont Circle will leave on the stroke of midnight.

Board member Franis Francois worried that "people will think midnight means just that, and will arrive at the station 14 minutes after the last train left."

Board member Joseph Alexander conceded that was possible, but said, "We'll have to let people know when the last train runs. We can put up some signs. They don't have to be all bronze and everything. Just signs."

The subway has run only on weekdays between 6 a.m. and 8 p.m. since it opened in 1976. On Monday, Sept. 25, weeknight operations will be extended to midnight. Saturday service will begin for the first time, from 8 a.m. to midnight. The board is studying the possibility of starting Sunday service later in the year.

Trains will operate at 10-minute intervals until 10 p.m., when they will switch to 15-minute intervals.

Bus schedules are being revised so there will be little parallel bus and subway service during the expanded subway operating hours.