The Southwest Louisiana Zydeco Music Festival, held in Opelousas every August, is the town's biggest event and draws visitors from across the region. August in Louisiana is hot and humid — expect temperatures above 35°C — but the music is excellent and the crowd is local rather than tourist. The Yambilee Festival in October celebrates the region's sweet potato harvest with food, music, and a parade, and is a more comfortable time weather-wise.
Spring (March to May) is generally pleasant and is the best time to visit for those without a specific festival in mind.