Anyone planning a big party is advised not to invite the New York Islanders.

The Washington Capitals staged a gala at Capital Centre yesterday, with NHL President John Ziegler presiding and pompons presented to a sellout crowd of 18,130 that produced a maelstrom of color and noise for the Centre's first Stanley Cup playoff game.

Then those curmudgeons from Long Island turned spoilsport, breezing to a 6-2 victory that left the Capitals one loss away from an unwanted vacation.

The Capitals now face the difficult task of beating the Stanley Cup champions here tonight (7 o'clock) and Tuesday at Nassau Coliseum to reach the Patrick Division final.

Bob Bourne and Bryan Trottier scored two goals apiece for New York and Roland Melanson stopped 26 Washington shots in goal as Billy Smith watched his first playoff game after 17 straight starts.

The major factor in the Capitals' demise, however, was their own power play, with power used in its idiomatic rather than literal sense. Washington managed not to score in 11 extra-man opportunities, which for seasonal futility topped the awful zero-for-eight effort on Long Island in March.

Worse, the Capitals twice permitted the Islanders to score a few seconds after player strength was equalized by relaxing with the expiration of the power play. Additionally, three New York goals came with each team a man short, as the Capitals seemed disorganized without that extra winger on the ice.

New York did not produce a shorthanded goal, but it came close. Denis Potvin was tripped on a shorthanded breakaway by Greg Theberge and awarded a penalty shot, only the 10th in Stanley Cup history. Potvin did not become the third player ever to convert one, because relief goalie Pat Riggin made the save.

"That was the first penalty shot I ever took in my life," Potvin said. "Thank God we were well ahead when it happened. I just wanted to come in and shoot, but there was an opening and I almost got it. It just hit the top of his pad."

Riggin, who replaced Al Jensen at the start of the third period with Washington trailing, 4-0, said he deflected the puck with his glove, so nit-pickers can take their choice. On one subject, however, nobody disagreed. The Washington power play was terrible.

"I'm going to decline penalties from now on," said Washington Coach Bryan Murray.

The Capitals this season ranked 18th in the NHL in power-play percentage, scoring 75 goals but failing on 283, as opponents took liberties with the knowledge they probably would not be forced to pay.

Asked why the Islanders' penalty killing, best in the NHL during the regular season, was so effective, Murray replied, "Because our power play is so bad. It's been a problem all year. We have no consistency. When we had a chance to break their box down, our shot from the point wasn't on net. When it was on net, the guy in front would be flaring left or right for an outlet pass. There's no doubt it discouraged our hockey team."

The first deflating incident, after the fans' tumultuous reception for their heroes pumped everyone up, came when the Capitals were presented with a two-man advantage for 84 seconds and failed to score for the 17th time in 21 such situations this season.

"This is a young hockey team, there was a lot of fan reaction and things were very emotional," Murray said. "We had a couple of unsuccessful power plays, the fans got on them and they started to press. I was on them, too--the fans have the same privilege."

Melanson had managed seven saves when Bourne produced the game's first score at 9:31 of the first period, on the Islanders' third shot. Washington's Scott Stevens poked the puck away from Bourne at the Capitals' blueline, but Mike Bossy retrieved it and fed Bourne alone in front for an easy score.

Each team was a man short at the time and the same situation existed when Bourne took a drop pass from Anders Kallur, who had lured Brian Engblom out of position, and made it 2-0 at 14:48.

Washington managed only one shot in the last 12 minutes of the period as its play disintegrated, and Trottier removed all doubt about the outcome with a pair of second-period goals, his third and fourth of the series.

Each came within a few seconds of the expiration of a Washington power play. The first followed some tentative play by Engblom and Stevens behind the Washington net, the second came while Murray was screaming at the power-play unit that had left the ice.

Third-period scores by Craig Laughlin and Scott Stevens were matched by John Tonelli, in another five-on-five situation, and by Bob Nystrom with less than two minutes remaining.

Laughlin, who converted a pass from Doug Jarvis on a play the two ex-Montrealers had concocted in practice, said, "When they got a goal in Long Island, we came back that much harder. Here when they scored it deflated our ego in front of the hometown crowd and we didn't come back. But we have all the confidence in the world. We're going to turn it around tomorrow and get a win for our fans."

Stevens' first Stanley Cup goal was spectacular, as he split the Islanders' defense and beat Melanson despite a hook by Potvin that brought an arm-jerk reaction from referee Bruce Hood.

"It's nice to get my first goal in the playoffs, but I'd rather win," Stevens said. "We'll come back tomorrow. That's the biggest crowd I've played in front of and when I came on the ice, I got chills. I hope we give them a better showing."

The pressure will not disappear tonight, because the game already is sold out. In sold-out home games through the years, including yesterday's, the Capitals have won two of 22.