Many people like the sailing to be around June 21 for the longest amount of daylight.
Cruises around that date tend to fill earlier.
We sailed the last week in July, first week in August, and experienced unusually warm weather with less rain.
Juneau, Skagway and Ketchikan, and of course Glacier Bay are popular. We enjoyed Seward, Sitka and Kodiak, and Victoria, B.C. was a very nice port. Hubbard Glacier was wonderful.
Excursions that were exceptional for us were the Sea Otter and Wildlife Quest out of Sitka, the White Pass/Yukon Scenic Railway out of Skagway, and the Deluxe Misty Fjord Flightsing Tour out of Ketchikan.