CONVENTIONS, usually referred to as cons, are the chief social events of science fiction. Here congregate writers, editors, readers, book dealers, button hucksters, T-shirt salesmen, all the elite and eccentric of fandom.

Younger fans will often sport elaborate costumes: You're liable to see Darth Vader, Princess Leia, Doctor Who, Conan the Barbarian, various crew members of the Starship Enterprise, and all kinds of caped crusaders, unearthly creatures and sloe- eyed slave girls. It's all part of the fun.

The huckster room -- more politely, the dealers' area -- will offer rare books, copies of Thrilling Wonder Stories, Planet Stories and other pulp magazines, Star Trek posters, hot comics like "The Shadow" or "Teenage Mutant Ninja Turtles" and assorted paraphernalia. How about a button for a divorced friend that reads "The Way to a Man's Heart Is With a Broad Sword"? Or a shirt that proclaims, "I've Been Seduced by the Chocolate Side of the Force"?

During the day a con will be organized around panels, some serious, most light-hearted. Depending on who's in attendance, you might hear scientist-writers David Brin or Jerry Pournelle talk about space weaponry; attend a discussion of "Heroic Lesbian Fantasy"; listen to Isaac Asimov reminisce about the early days of science fiction; get tips from editors like David Hartwell or Terry Carr; attend writing seminars or master classes with famous illustrators. You can visit the art show, see the original paintings for dust jacket covers, buy or bid on watercolors, drawings and sculpture. At midnight you'll want to check out the midnight horror panel, usually devoted to dissecting the worst horror films ever made ("Plan Nine From Outer Space" is nearly everybody's favorite).

During the evening, the con suite will usually host a sock hop, provide beer and light refreshment. (One of the deejays might be Gardner Dozois, editor of Isaac Asimov's Science Fiction Magazine and the funniest man in the galaxy.) After 9 or 10 p.m., private parties will begin all over the convention hotel; most are casual affairs and anyone can drop in. If you don't feel like socializing, you can wander over to the movie room where classic and classically awful sf films are shown throughout the day and half the night.

On one evening of the con there will generally be a masquerade. Some fans become intensely interested in creating original sf costumes or in carefully re-creating the garb of figures from sf movies and books. The official masquerades can be quite professional; indeed many of the designers and models work in theater, film or the fashion industry. Some of the outfits are elaborate beyond belief; others deliciously skimpy. A masquerade is something like a blend of Jabba the Hutt's den, a Las Vegas revue and an old-fashioned costume party. Don't miss it.

Mostly though, cons are weekend-long bull sessions about science fiction. Writers sit in the bar talking about agents and contracts; fans lounge in the hallways arguing about novels and movies; both groups get together at parties to compare this con to last year's, to sing silly songs and to talk about fandom, publishing, hot new writers, favorite books. There's nothing quite like it, except life in a large, diverse and very affectionate family.

BALTICON -- Gas up the car and head north, right now. Held each year at Easter, Balticon is taking place Friday (4 p.m.) through Sunday (4 p.m.) at the Hyatt Regency Inner Harbor. Membership for the three-day meeting is $25 and may be paid at the door. Guests of honor are writer Nancy Springer ("The Golden Swan" and other books) and fantasy artist Vicki M. Wyman. Other pros attending include Gardner Dozois, Marvin Kaye, David Bischoff, Somtow Sucharitkul, David Hartwell, Tim Sullivan, Ray Ridenour, Donald Kingsbury. Masquerade Friday night, emceed by Count Dracula (noted costumer Marty Gear). Compton Crook award for best first novel of the year. Dealer's room. Writer's workshop. Poetry readings and filk singing (science fiction folk songs). Performance of Joe Mayhew's play "Crosstime Bus." Movies and videos shown from 8 a.m till 4 a.m. Panels include "Bangs and Whimpers: The Real End of the World," "Future Sex: The Human/Nonhuman Interface," "Horses in Fantasy and Reality," science programming by members of the L-5 society, and a late night joke exchange. Parties and programming originate at the Green Room. Hosted by the Baltimore Science Fiction Society. For information: Telephone 301/889-3290.

DISCLAVE -- May 23-25 at the Sheraton Inn Hotel in New Carrollton. Writer guest of honor will be William Gibson, whose first novel "Neuromancer" made a clean sweep of sf literary prizes in 1984. Artist guest of honor is Steve Stiles of Baltimore. Workshops, panels, speeches, more. Membership in the con is $15 until May; $20 at the door. For information, write to Disclave, P.O. Box 971, College Park, MD 20740, or telephone Dolly and Alexis Gilliland at 920-6087. Members are asked to bring baked goods as contributions to the International Cookie Conspiracy.

UNICON -- Generally takes place in Silver Spring on the third Sunday in July, but this year has been canceled because of problems in finding a hotel. Keep your calendar free for July 1987. CONFEDERATION/WORLDCON 4 -- August 26-September 1, in Atlanta. It's only occasionally in Washington (last year it took place in Australia), but most D.C. trufans nonetheless try to make the Worldcon, the high point of the science fiction year. Generally from 6,000 to 8,000 fans attend; vote on the Hugo awards for the year's best fiction, art and films; listen to speakers, go to parties, dress up in costumes and make merry. This year the guest of honor is Ray Bradbury; fan guest of honor is editor Terry Carr, whose Ace specials have introduced many of the best new writers. Membership is $65. For information: Confederation, Suite 1986, 3277 Roswell Rd., Atlanta, GA 30305.