There are two major festivals in Vieques [1]. The biggest one is Fiestas Patronales de Nuestra Sra. del Carmen (787/741-5000), which is held on the plaza of Isabel Segunda Wednesday–Sunday during the third weekend of July. Attractions include parades, religious processions, a small carnival, and lots of live Latin music and dancing. Entertainment usually starts around 9 p.m. and lasts until the wee hours of the morning. Festivities are fueled by bili, a traditional beverage made from a local fruit called quenepa mixed with white rum, cinnamon, and sugar.
The other big event is the Cultural Festival (787/741-1717), sponsored by the Institute of Puerto Rican Culture at El Fortín Conde de Mirasol [2] in Isabel Segunda after Easter. Festivities include folk music and dance performances, a craft fair, and a book fair.
Links:
[1] http://www.moon.com/destinations/puerto-rico/vieques-and-culebra/vieques
[2] http://www.moon.com/destinations/puerto-rico/vieques-and-culebra/vieques/sights/el-fortin-conde-de-mirasol