Avignon is best avoided in the French school holidays, and ideally in the UK holidays too, as it is a popular destination with British families and gets rather crowded. Being in southern France, it is very hot in July and August - which, apart from also being the busiest times, is another drawback if you don't like the heat much. It also gets the mistral (a wind originating from Africa) at many times of the year, which can be unpleasant.

From about mid-September Avignon gets quieter, though it is fairly busy all the year round. The mild climate also makes winter breaks possible - although it can be rainy, it isn't usually too cold apart from the wind. There are also events going on all year round, so it's worth having a look at the events calendar and seeing if there is a concert, exhibition or other show that would interest you.