THE major mental activity of Spring Weekend is pretending. To begin with, you have to pretend that Spring Weekend exists. You may notice that some of your classmates are not observing the event with proper jubilation; or not observing it at all. Some of the more cynical will even tell you that The Weekend is all a money-making plot on the part of some Harvard organizations. Do not believe them.

If you can make this great mental leap forward, it is possible to pretend that there will be some athletic competition this Saturday when four Harvard varsity teams play at home. This will take some effort though. The zany unpredictability that usually enlivens Harvard sporting events is unlikely to enter any of this weekend's.

The baseball team always puts on an exciting exhibition. If the team's pitching has been good this Spring, its hitting has sometimes been unnoticeable and its fielding has turned what looked like Crimson routs into exciting contests by giving away runs.

But this weekend, inexcusably, the team has simply pulled up its stakes and departed from Cambridge, fleeing Spring Weekend for the less hospitable climate of Philadelphia and Ithaca, and taking with it one of the few sparks of athletic excitement in the neighborhood.

Even without the Harvard baseball team, there are usually the Red Sox, just a short MTA ride away. The Sox can provide excitement any time, rising up to defeat the omnipotent. Yankees, or blowing ball games to Washington and Kansas City. But they, too, will flee Boston this weekend.

What's left? Well, there's the Yale track meet, for two years a thrilling event which has been decided by the final relay. A vivid imagination might even be able to foresee a close meet starting at 1 p.m. Saturday.

After all, the eli track team boasts Wendell Mottley, a quarter-miler from Trinidad who sets more world records than anyone can keep count of (he set one this winter running in sneakers on a gym floor at Cornell). Mottley is guaranteed to make a shambles of the quarter mile this afternoon, and the Yale team he anchors is a cinch in the mile relay.

But a Harvard team that boasts no Mottleys is more than able to cope with the Trinidadian's challenge. Ed Meehan has never set a world record while running in his sneakers, but he is a strong, courageous, exciting runner who should win the 880 and the mile. Yale has no sprinter who can stay with Aggrey Awori, no hurdler in a class with Tony Lynch, no two-miler who can challenge Walt Hewlett on a good (though the Elis' Ross o Dell has caught Hewlett on bad days before). If Yale's pole vaulters and javelin men are fairly sure winners there is no Bulldog who can leap with Chris Ohiri, a consistent 23-footer in the broad jump and a threat to go right out of the pit in the hop, step and jump. The close events should be the shot put, where Chuck Merecin and Art Croasdale renew an old rivalry, the discus, where Harvard's Heps champion John Bakkensen and George Levendis are about even, and the high jump, where two amazing sophomores, Harvard's Chris Pardee and Yale's Kim Hill, are consistent at 6-6. But the Crimson's strength in the track events should decide the meet easily.

IT takes an even more vivid imagination to envision a close race on the Charles, where the Crimson faces Penn and Navy in the Adams Cup races on Saturday afternoon. Harvard's heavyweight crew is good, but it probably isn't in a class with Navy, a favorite to represent the U.S. in the Olympics.

The Harvard crew is in a strange predicament. It is big, strong, and powerful, which is usually enough to guarantee a winning boat. This year, though, a sprinter's speed is also needed because all races are at the Olympic distance of 2000 meters. If the meets were all at 4 miles, the Crimson might take everything. But there is some doubt whether Harvard can work up enough speed to win at the shorter distance.

If wishful thinking is needed to envision a close crew race, it would take a visionary to look for competition. when the Harvard tennis team faces Navy at 2 p.m. Saturday. Last year the Crimson won 7-2; since then Lee Pekary, the little dynamo who accounted for both Navy points, has gone off to sea, or whatever Midshipmen do when they graduate. That leaves a depleted Navy team to face an excellent Harvard squad.

Frank Ripley, the Crimson's top player, plays a picture game, adding smooth ground strokes to a style that depends on a big serve and volley. At 6-3, Rip has the power to hit with anybody in the Eastern League, and he should have more than enough for Navy.

Behind him, Chum Steele is a top-calibre doubles player who works with fanatical energy and hopes to become one of the country's best. On a good day, Steele can beat anyone in the Eastern League; his problem is erratic play, but even a bad day probably wouldn't cost him the match against Navy.

Two sophomores, Clive Kileff and David Benjamin, plus captain Sandy Walker and Bob Inman, round out a squad that might easily shut out the Midshipmen 9-0.

If there is to be a close game, it will come on the Business School Field at 3 p.m. Saturday when the erratic Harvard lacrosse team faces Dartmouth. The Indians are a sophomore-dominated team, and probably much weaker than the squad that beat Harvard 13-7 last year. But the Crimson attack has sputtered and coughed all year, and may not be in top shape today. The defense, too, has had trouble clearing the ball.

Harvard's sporadic punch has come largely from a first midfield centered by Charles (Tink) Gunnoe, one of the most colorful athletes at Harvard this spring. Gunnoe is ambidextrous, and his shot from either side is hard and straight. His stick handling is a joy to watch even if you don't know much about lacrosse. With his running mate, Peter Wood, he has provided most of the excitement for the lacrosse team this year.

On a good day, the lacrosse team should have no trouble with Dartmouth; there's surely little doubt about the other three games, so you may be in for a dull time no matter what sports event you watch. But at least you'll have a seat, hopefully in the sun, with a girl beside you. And that's what Spring weekend is all about anyway