The Montreal Canadiens' decision to begin the Stanley Cup final on Sunday at 4 p.m., instead of Saturday night, means that Washington hockey fans will not see the opener of the best-of-seven series.

WDCA-TV-20, which had planned to carry the game Saturday night, is committed to show the movie "Green Berets" Sunday afternoon.

"We have commitments and we have advertising sold specifically for the Sunday afternoon movies," said Jim Reid, WDCA program director.

"My hands were sweaty when I took the call this morning on the switch in game time. They were sweaty not because it's any problem to fill Saturday with a movie, but because it means all day Monday on the phone with angry hockey fans."Hockey fans are persistent. I just wish there were more of them. The hockey has presented a problem for us. We do have an agreement to do the Caps' games and we plan on continuing in their corner, but how far can you go trying to work with it?"

Saturday is the usual night for Montreal home games, for Hockey Night in Canada telecasts and for the NHL network games. However, the home team during the Stanley Cup final can choose to play its games on either Saturday or Sunday.

Because it played a tough game against Boston Thursday and the New York Rangers had been idle since Tuesday, the Canadiens decided to delay the start until Sunday. They preferred Sunday night, but were blocked by television, since a nationally televised debate among Canada's three party leaders is scheduled Sunday night, nine days beforee the election.

To make the switch to Sunday afternoon, the Canadiens invoked a clause in their contract with the Memorial Cup, Canada's junior championship, which was scheduled in the Forum Sunday, and forced that contest to be played in nearby Verdun Auditorium.

"It's a good move with the injuries we had," said Montreal Coach Scotty Bowman. "You play all year and rush and rush and rush. When you get to the final, what's another day? This will give us an extra day of practice.

"We first thought about 5 p.m., but we were afraid if it went into overtime we'd have another Heidi situation-in Canada."

Because of the election, Game 5 will be played here Monday, May 21.

Channel 20 will televise Game 4 from New York on May 19 at 9 p.m. Reid promised that it will also try to clear time for a tape delay of any games that could terminate the series, that is, after one team has gained three victories. CAPTION: Picture, Serge Savard, left, raises the arm of goal-scorer Yvon Lambert, who ousted the Bruins. UPI