Originally Posted by CanadaDH

September and June are both great months in Lake Louise. Which is better for you, depends on what you plan to do. Next to July and August, June and September are the most popular months there for tourism, so it must still be good!

June has the advantage of longer daylight hours, and scenery with a lot of snow still capping the mountains. You can tour for much longer in the day. It also has the disadvantage in that the alpine hiking trails will still have snow, and some may not be passable. If you plan is to mainly stay at lower elevations, then snow on the trail will not be a concern. I wouldn't worry about ice on Lake Louise by mid-June. Historically, the ice off is around the first week. It ranges from late May, and the record in recent years for the latest ice-off date is June 13. Higher elevation lakes, like Moraine, Agnes, Peyto or Bow, may hold their ice a week or two longer.

September is statistically a drier month, so better chance of clear skies for hiking, and less snow on trails. However, by the start of September, you will start to get fairly cold weather at higher elevations, and some snow blowing in (which typically doesn't last on the ground too long). Mid-September is still a bit too early for the changing of the larch trees, but it does vary each year depending on the weather. You might see some early signs of changes, but the tail end of the month is more likely to have the bright orange colours.

Personally, I'm going in mid-August this year. I even managed to score Lake O'Hara bus tickets for this trip after years of trying. I finally get to hike the famous Alpine Circuit there!