Friday evening through Sunday mornings finds the Mid-Atlantic Chapter of the American Theater Association in convention at the Little Theater of Alexandra.

The setting couldn't be a better example of ATA dreams, for the Alexandrians built their own well-equipped theater from performance profits that accrued over 40 years. Such community theaters are scattered across the land, but the LTA set its building aims many years ago and never fattered.

Friday night will see a tour of the theater before excerpts from three LTA productions, Pinter's "The Lovers," "Oh, Coward!" and "1776." On Saturday, starting with 8 a.m. registration, will come a full day of workshops, panels and special performances into the evening. At a 10 a.m. brunch Sunday, director Alan Scheider will be the guest speaker.

The Theater School's annual one-day seminar at the Kennedy Center will be Friday, Nov. 4, from 10 a.m. until 3 p.m., an intensive period for which the principal speakers will be Roberta Maxwell, the glowing Portia of the Eisenhower's "The Merchant," producer Richard Barr and director-critic Harold Clurman. This annual event is planned for high school juniors and seniors, teachers, guidance and career counselors and the general theater audience. Registrations, which must be settled by Oct. 28, may be made by phone at 638-2813.

Trazana Beverley and Oz Scott, the "Lady in Red" and the director of the National's "For Colored Girls . . ." will talk of that novel work Wednesday noon in the Kennedy Center's weekly symposium in the AFI Theater. With Sam Johnson as moderator, it will be aired live on WETA.

Saturday, Sunday and Monday this Veteran's Day weekend will bring a series of plays, puppet shows, dance and music concerts to the Streets for People area in the blocks bounded by F.G. 7th and 10th Streets, NW. Mayor Washington and city officials start the events Saturday at 2 p.m., with Peter Share as coordinator for City Celebration.

Performing in French, Le Treteau de Paris company will play Ionesco's "La Cantatrice Chauve" ("The Bald Soprano") Sunday afternoon at 3, evening at 8 in George Washington University's Marvin Center. Nicolas Bataille, who staged the original Paris production in 1950, heads the cast, which will precede the Ionesco with songs and sketches by Jacques Prevert. Phone reservations at 676-6178.

To Ralph Redpath, out of Yale drama school only two years, falls the rich part of "Mister Roberts," which follows Virginia Graham and "Any Wednesday" at the Hayloft Dinner Theater for a four-week run starting Tuesday. Vincent Clark will be Pulver and Arn Weiner the captain.

"The Passion of Sacco and Vanzetti," as performed by the New York Street Theater Caravan, will have a single performance Friday night at 8 in American University's New lecture Hall. Directed by author Marketa Kimbrell, the cast includes former AU actors Ron Litman, Doug Hudgins, Ted Hannon and Cindy Wright, further information at 686-2317.

The Edwin Booth Theater's ambitious production of "Oliver!" is underway at Harford Community College with a cast of 85. Performances are Wednesdays through Sundays through Nov. 6 at 401 Thomas Run Rd., Bel Air, Md.; phone information at 879-8920.