Terrassa is at its best in late spring and early autumn, when temperatures sit between 18 and 26°C and the parks are at their greenest. Summer (July to mid-September) is hot — frequently over 32°C — and many local restaurants close for two to three weeks in August.
The Festa Major in early July is the city's biggest event, with parades of giants, castellers (human towers), correfoc (fire-runs) and concerts spread across the old town squares. The Jazz Terrassa festival in March is one of Catalonia's most respected jazz events, drawing serious international acts. Avoid the local industrial fair in October only if traffic is your main concern; otherwise the city handles it well.