The Cinco de Mayo Festival is where I'll be heading to get in a Latin mood during the daytime before the night's festivities. Taking place at the National Mall's Sylvan Theater (15th Street and Independence Avenue, SW) from noon until 6pm, the festival features everything from a dance troupe from Panama to pinatas to mariachi groups. Free.
If music's what you want to get down to on the 5th, there'll be nowhere better to head than the State Theatre (220 N. Washington Street, Falls Church), where three bands team up for a great show. Spanish language area rock groups Machatres, Stone Gato and Nayas play starting at 7pm, and at just $10 for a ticket, you'll have plenty of money leftover for margaritas later.
If Cinco de Mayo for you translates into "drinking as many margaritas as possible," then you should get on the train for Cinco de Midtown, a bar crawl taking place at Madhatter, Porter's, the Front Page, Rumors and the Overtime Cafe. $5 at any of the bars gets you a wristband for entrance into any of them, with $2 beer specials and $3 margaritas. 5 pm to 10 pm. Details and location info here.
Professionals in the City is renting out Cecilia's (2619 Columbia Pike) in Arlington, a salsa spot and bar. $12 gets you salsa lessons, a dance contest, and live salsa and merengue music. 7:30 pm to 10pm.