Oropesa works year-round for a day visit, but spring and autumn are the most pleasant. The town's Medieval Days festival in April is the best reason to time a trip: markets, costumed guilds, and jousting fill the castle precinct and the streets below for a weekend. It is a genuine local event rather than a tourist confection and well worth catching.
Summer is warm to hot — daytime highs above 35°C are common in July and August — and the castle and old town offer little shade. Winter is mild and quiet; the parador and castle are open all year, and the surrounding countryside is green with winter rains. The Sierra de Gredos above the town holds snow from December to March, which makes for a dramatic backdrop.