Spring (April to early June) and autumn (September to October) are the best times to visit Certaldo. The weather is mild, the Val d'Elsa countryside is at its most attractive, and the hilltop town is pleasantly uncrowded. Summers are hot — Tuscany in July and August can push 35°C — and while Certaldo is far less overrun than Florence or Siena, it does get busier.
The highlight of the summer calendar is Mercantia, an international street theatre festival held each July in Certaldo Alto. The entire upper town becomes a stage: jugglers, acrobats, theatre companies, and performers from across Europe fill the medieval streets for several days. It transforms the town entirely and is worth timing a visit around if you enjoy that kind of thing. The narrow lanes of Certaldo Alto make an ideal backdrop. Book accommodation in the area well in advance if you plan to attend.
August is the peak holiday month in Italy, and while Certaldo is not a mass-tourism destination, many local restaurants and businesses take their own holidays during this period. September brings grape harvest season to the surrounding vineyards and is a fine time to be in the Val d'Elsa generally.