Saturday is one day shy of the 20th anniversity of Abraham Lincoln'sassassination at Ford's Theater, and the opening day for a month-long exhibition, "Lincoln and the American Political Tradition." This impressive collection of archival material includes poems written by the 16th president, letters, manuscripts, newspapers, photographs, illustrations and medals from Brown University's John Hay library. Beginning at 1, noted historians will speak on a variety of Lincolniana topics and will present a special multi-media show. It's free, at Ford's Theater, 511 10th Street NW, open 9 to 5 daily except during rehearsals and matinees on Sunday and Thursday afternoons. Call 426-6924. *Saturday is also the 242nd anniversary of the birth of Thomas Jefferson, our third president, author of the Declaration of Independence, founder of the University of Virginia and inventor. A wreath-laying ceremony will commemorate the day at the Jefferson Memorial at noon. 426-6690. *On several occasions before it became the Boyhood Home of Robert E. Lee, George Washington slept at 607 Oronoco Street in Alexandria. All day Saturday, from 10 to 4, Washington's visit to the home of Colonel William Fitzhugh will be recreated, complete with period music. Adults $2, and students, $1. 584-8454.