Summer (June–August) is the best time for Vitebsk: warm weather, long daylight hours, and the Slavianski Bazaar festival in July, which brings life and noise to a city that can otherwise feel austere. Winter is cold and grey and the city turns inward. Spring and autumn are mild and uncrowded — good for quiet visits to the museums and riverside walks without the July festival crowds.
If you plan to attend the Slavianski Bazaar, book accommodation in advance; the festival runs for about a week in mid-July and hotels fill quickly.