Warwick is at its best from late spring through early autumn, when the castle gardens are full and the light catches the sandstone walls. July and August are the busiest months — the castle gets very busy at weekends and the queue can take an hour at midday. Visit early in the day or on a weekday if you can.
Winter has a different appeal: the town is quiet, the castle hosts Christmas events from late November, and an evening walk through the cobbled lanes by lamplight is genuinely atmospheric. Most of the smaller attractions reduce their hours from October to March, so check websites before counting on them.