Two seasons matter: crawfish season (roughly January to early June) and the festival calendar. Crawfish are at their best in the spring; outside the season most restaurants serve frozen tails rather than live boils.
The big events anchor the year. The Festival International de Louisiane in late April draws Francophone musicians from around the world for five free days in downtown Lafayette. Festivals Acadiens et Créoles in mid-October celebrates Cajun and zydeco music at Girard Park. Both are worth planning a trip around. Mardi Gras in Lafayette is more family-oriented than New Orleans, with daytime parades and the chicken-running Courir de Mardi Gras in surrounding villages.
Summer is hot, humid, and sticky. Spring and autumn are the most comfortable months for outdoor time.