When to come — and when to absolutely avoid
By season
Quietest: Mid-January to early March and the first three weeks of November. English winter weather isn't glamorous, but the State Apartments are wonderfully warm, the Semi-State Rooms are open, and you can wander St George's Chapel without a queue. The shorter daylight hours also mean dramatic late-afternoon light on the Round Tower.
Busiest: July and August (peak summer holidays), the Easter weekend, English school half-terms (mid-February, late May, late October), and the entire week between Christmas and New Year. Saturdays year-round are noticeably busier than weekdays. Avoid the Saturday closest to St George's Day (23 April) and the second Monday in June — the Order of the Garter Service often closes the State Apartments entirely.
By day of the week
Thursday and Friday mornings are calmest. Saturdays are always busy with day-trippers from London. Mondays can be surprisingly quiet because many overseas tour operators assume the castle is closed (it isn't). Sundays have a different crowd — no State Apartments queues until later in the day, and the chapel is open for worship rather than tourism.
By time of day
The first slot at 10:00 and slots after 14:30 are noticeably quieter than the 11:00–13:00 peak when most coach tours from London arrive. Aim for the very first slot of the day in summer; aim for late afternoon in winter to catch sunset light on the Round Tower from the Lower Ward.