The Washington Bullets put on one of the most courageous performances in the history of the franchise tonight, right there beneath all those championship banners in Boston Garden, as they pushed the mighty Celtics to the edges of their thrones before finally losing in two overtime periods, 131-126.

The victory advanced the Celtics to the Eastern Conference final, Boston having beaten the spunky Bullets, four games to one, in this best-of-seven series. The Celtics will face the winner of the Philadelphia-Milwaukee semifinal in the next round. The Bucks beat the 76ers in Philadelphia tonight, 110-98, to force a sixth game in that series Friday in Milwaukee. The 76ers lead the series, 3-2.

The Celtics weren't thinking about the 76ers, Bucks, Lakers or any team but the Bullets tonight as the upstarts and castoffs from Washington kept picking themselves up off the floor and going back after the defending National Basketball Association champions. They weren't intimidated by the Celtics or the screaming sellout crowd of 15,320 here, not even when they were blitzed by a 24-6 Celtic burst in the third quarter that left them 18 points behind.

Three incredible 32-foot three-point shots by Frank Johnson in the final three minutes of regulation enabled the Bullets to eliminate a 10-point deficit in that span. His third three-pointer, with 41 seconds to play, got the Bullets to within two, 106-104.

With Greg Ballard guarding him, Larry Bird missed a jumper and Jeff Ruland, who had a career-high 33 points, got the rebound and was fouled by Robert Parish. Ruland's two free throws with 24 seconds left tied the game at 106 and sent it into overtime.

Parish missed a jumper and Ruland got the rebound with one second left.

There was some confusion in the last second as to whether the Bullets had called timeout. Lead official Jake O'Donnell, amid the frantic pleas of the Bullets, who said they had not called time, ruled that they hadn't, and the game went into overtime.

Because of Johnson's three-pointers, the Bullets outscored the Celtics, 16-3, at the end of the fourth quarter, getting those 16 points on their last seven possessions.

The Bullets had the Celtics reeling in both overtimes, but couldn't clinch the victory.

"I thought we should have won, but that seems like the way it always is," said Bullets Coach Gene Shue. "It was a fantastic ball game, but . . . "

After a 15-footer by Ballard and a layup by Ruland around a miss by Parish, the Bullets had a 117-112 lead with 1:20 remaining in the first overtime. Spencer Haywood fouled out, resulting in Cedric Maxwell making two free throws with 61 seconds left. Kevin McHale, overplaying Rick Mahorn, stole Ballard's pass and drove three-quarters of the length of the floor for a layup to get Boston to within a point, 117-116, with 44 seconds left.

Mahorn put the Bullets ahead by three with a 15-foot jumper, and Archibald made it 119-117, making one free throw after being fouled by Johnson with 21 seconds remaining.

Archibald fouled Ruland, but the Celtics weren't in a penalty situation yet, so the Bullets had to inbound the ball at midcourt with 10 seconds left. Ballard inbounded it to Johnson two feet in front of him, and Bird reached around and tied up Johnson, forcing a jump ball at midcourt.

Bird won the tap, getting the ball to Archibald. His running, one-handed, offbalance jumper bounced off the glass, far from the rim, but McHale came leaping in from the left side, grabbed the ball and put it in--all in one motion--to force the second overtime.

"We forced the bad shot, just like we wanted to, but then we let them get the rebound," said Shue. "That killed us."

The Bullets scored first again to start the second overtime as Ballard made a turnaround jumper. After Bird tied at 121 on a drive, Johnson made two free throws, followed by two more by Ruland, and the Bullets had a four-point lead, 125-121, with 2:50 left.

Maxwell scored on a driving layup, and after Ruland's layup was blocked by Parish--his sixth block of the game--McHale scored on a running hook, tying at 125 with 1:55 remaining.

Johnson was fouled by Gerald Henderson, but made only the second of the two free throws for a 126-125 Bullet lead.

Washington never scored again.

Johnson forced Henderson to his left, as he had been instructed, got no weak-side help on defense when Henderson went that way, and the Celtic drove 30 feet for an uncontested layup and a 127-126 Boston lead.

On the Bullets' next possession, Johnson's jumper was blocked by Henderson. The Celtics then got three offensive rebounds before Parish finally scored as he was fouled by Mahorn. His ensuing free throw gave Boston a 130-126 lead, and after Johnson missed two more three-point attempts, Maxwell made one of two free throws to account for the margin of victory.

Bird and Maxwell finished with 26 points each, and Johnson had 22 and 10 assists for the Bullets. Ballard had 18 points, 12 rebounds and five steals.

Kevin Grevey started for Don Collins, who had a sprained ankle, and scored 10 points, but all of them were in the first half. Collins played only two minutes and didn't score.

This was the second straight overtime loss for the Bullets in this series and the second game in which a near-brawl occurred. This time Johnson and Rick Robey almost came to blows in the fourth period, but even though both benches cleared, no punches were thrown and no technical fouls were called.

"There was no way we were going to concede anything to Boston," said Haywood, who scored 21 points. "We came up here to win, not just to look good. We never quit, because we always felt we could win. The Celtics got us this time, but next time will be different."