St. Andrews has a short, intense season. July and August are the warmest months — daytime highs around 22°C, water temperatures cold but swimmable for the brave — and almost everything is open. The town gets busy without becoming oppressive, and whale-watching trips and Ministers Island crossings are at their most reliable.
June and September are quieter and arguably more pleasant: the gardens are at their best in mid-June, the fall colours arrive by late September, and accommodation is easier to find. October sees most attractions wind down and many restaurants close.
Winter is genuinely sleepy. The Algonquin and a handful of pubs stay open, the ice on the bay can be beautiful, and the population drops to its core of about 1,800 residents. Storms blow through; expect closures and short days. If you want St. Andrews at its most evocative, mid-September is probably the sweet spot.