Hallstatt is busy from May to September and on weekends year-round. The most pleasant months are May, June, and September: long days, full waterfalls, the high mountain paths just opening up, and noticeably fewer coach parties than in July and August. October brings autumn colour to the larch forests, and from late November to early January the village hosts a small but very pretty Christmas market on the Marktplatz. January and February are cold and quiet — the salt mine closes, the funicular runs reduced hours, and the village largely shuts down — but the snow-on-rooftops view from the classic viewpoint is unforgettable on a clear day.
In peak season the village is overrun between roughly 10am and 4pm. Stay overnight if at all possible: in the evening and early morning you will have the place largely to yourselves.