By Rick Steves
Sea breezes off the Mediterranean and a generally warm climate make Barcelona pleasant for much of the year. Late spring and early fall offer the best combination of good weather, lighter crowds, long days, and plenty of tourist and cultural activities. You'll encounter hot, humid weather and the biggest crowds in July and August, and some shops and restaurants close down in August. Winter temperatures are far from freezing, but rainfall is abundant.
Barcelona always feels festive, but it's especially fun during carnival season in winter, and on St. George's Day (April 23), when it celebrates its patron saint (a.k.a. Sant Jordi) with flower markets and book stalls along Las Ramblas, and colorful sardana dances on Plaça Sant Jaume. Nearby Berga hosts the firey religious festival called "La Patum" in mid-June, and local pride is at a peak on Catalunya Day (September 11). And a week or two later comes all the street theater, music, and processions — both daytime and nighttime — of La Mercè.
For a longer list of celebrations, see Upcoming Holidays and Festivals in Spain.