Blois is at its best from late April to early October. Spring brings green river meadows and the first comfortable warm days; September and early October hold the light well and have fewer tour buses. Summer is the busiest season, especially around the château and on the bridge — book accommodation and Orangerie du Château well ahead in July and August.
The summer son et lumière at the château (April to September) is genuinely worth timing a visit around; check the schedule before booking. Christmas is small but pleasant, with the Escalier Denis-Papin used as an outdoor projection screen for evening light shows.
Winter is quiet to the point of being deserted, which has its own appeal. Many smaller restaurants close in January and February. The cathedral, château, and the nearby châteaux are all open year-round.