We cruised late Sept. last year, in fact over Labor day week-end. Weather can really be almost anything, and expect some rain along the way, pack som rain gear, or plenty of the stores sell the cheap poncho's for $10. Just pack and dress in layers, we actually had a few afternoon's in the 70's, so throw in at least 1 pair of shorts.
There is a good Alaska cruising guide, here is the link:
I also wrote a review of Celebrity Infinity with pics at the end:
Have a great Alaska cruise!