Kutná Hora is heavily visited in summer, especially July and August when tour buses from Prague arrive daily and the main sights can feel crowded by mid-morning. Spring and autumn are more pleasant: the ossuary and churches are no less impressive and the town is noticeably quieter. A weekday visit at any time of year is better than a weekend.
Winter is perfectly fine for a visit — the town is quiet and the cathedral looks particularly good under grey skies — but confirm opening hours for the ossuary and museum, which may have reduced hours from November to March. The St. Barbara's Church and Italian Court are open year-round.