Tomar has a typical central Portuguese climate: hot, dry summers with temperatures regularly above 35°C, and mild, often rainy winters. Spring (April–June) and early autumn (September–October) are the best times to visit — warm enough for outdoor sightseeing, not yet overwhelmed by the school-holiday crowds that descend on the convent in July and August.
The town's most famous festival is the Festa dos Tabuleiros, held every four years (years divisible by four) in July. It is a major event involving a procession of hundreds of young women carrying enormous headdresses of bread and flowers on their heads — the headdresses can be taller than the bearer. The festival draws enormous crowds and accommodation in the town fills up months in advance. Outside festival years, July and August are still busy but manageable.
If you are primarily interested in the convent, arrive at opening time or in the late afternoon; the middle of the day in summer is both hot and crowded. The convent is open year-round except for a handful of public holidays.