Strasbourg is excellent year-round, but the Christmas market (late November to December 24) is a serious draw — one of the oldest in Europe, spread across six different squares including the cathedral forecourt and Place Kléber. The crowds are dense but the atmosphere is genuinely magical. Book accommodation months in advance.
Summer (June to August) is pleasant and warm; the canals of La Petite France are at their best. Spring is quiet and the gardens are green. Winter outside the Christmas market period is cold but uncrowded — a good time to visit the museums without queuing. The European Parliament sits in plenary session about once a month, which brings a rush of MEPs and journalists but doesn't affect tourist facilities.