Callander is busiest in July and August, when the Trossachs fill with visitors and the main street can feel crowded. The weather is also at its most reliable then, though "reliable" in the Scottish sense means fewer guaranteed dry days rather than guaranteed sun. Midges — the tiny biting flies that make Highland life miserable — are at their worst from June to August, particularly near still water and in the early morning and evening.
May and June are often the best months: long daylight hours (sunset past 10pm in midsummer), wildflowers in the woodland, and the hills clear of their winter snow without the August crowds. September and October bring colour to the birch and oak woodland and markedly fewer people; the hills are still walkable, though snow can arrive early on the higher ground.
Winter has its own appeal — Ben Ledi often has a dusting of snow, the light is extraordinary in the low sun, and Callander itself is quiet enough to feel like a real place rather than a tourist processing centre. The Callander Jazz and Blues Festival takes place in August and is worth timing a visit around if music is your thing.