The snow begins to melt, the sun warms the air, but evening temperatures can still drop to –10º C or colder. And it can still snow. You can still get frostbite. Keep your winter gear at hand, but add a lighter jacket and gloves. Keep your winter hat handy. Dress in layers and adjust accordingly.
Our high season. Temperatures are typically in the comfortable mid 20sºC, though it can get hot. It rains occasionally and the odd spectacular lightning storm, which ends about 10 minutes after it starts. It can be cool in early morning, so if you're heading out on your bike or hiking, you might want some full coverage biking or knit gloves. In the alpine, you might also want a toque. Light sweaters or jackets, a rain coat or umbrella, good walking shoes or hiking boots and some summer sandals are musts in Banff.
The Larches turn golden and the temperatures fall toward the end of September. Be prepared. It can rain. It can snow. It can be brilliantly sunny and 20º C. All on the same day. Dig out your woollies.