The season is shorter than people expect
June through August is the real Alaska season, with a little either side in late May and early September. That is it. Everything, the ships, the lodges, the rail cars, the guides, is compressed into about fourteen weeks.
Two things follow from that, and both cost money if you learn them late.
The good inventory goes early. Balcony cabins on the sailings with the right permits, and rooms near Denali, are booked out a long way ahead. Alaska is the trip I tell people to lock a year out, and I mean it more here than anywhere except a river cruise.
The shoulder weeks are a genuine trade, not a bargain. Late May and early September are quieter and cheaper. They are also cooler, wetter, and some excursions and lodges are not yet running or have already stopped. If you want the long daylight and the full operation, come in the middle.
What the light actually does
In June, the far north barely gets dark. That sounds like a novelty and it changes the trip. You will be out later than you intended, wildlife is active at hours that surprise you, and if you sleep badly in daylight you will want a cabin you can genuinely black out.
By late August the nights are back and the northern lights become a possibility in the interior. Those are two different vacations. Neither is better, and it is worth knowing which one you are buying.
The weather conversation, honestly
Southeast Alaska is temperate rainforest. It rains. Anyone who tells you otherwise is selling you something.
Pack for layers and rain rather than for cold: a proper waterproof shell, something warm underneath, and shoes you do not mind soaking. Most days on deck are in the fifties. The rain is not a problem unless you arrive expecting sunshine, in which case it is the whole trip.
The mistakes that cost people the trip
Booking the cruise and assuming Glacier Bay is included. It very often is not, and by the time you find out, the sailings that do have the permit are full.
Leaving the land portion until after the ship is booked. The lodges and the rail seats have less capacity than the ships do, so the land side should shape the cruise dates rather than the other way round.
Treating the flights as an afterthought on a one-way sailing. Flying home from Anchorage after boarding in Vancouver is a different piece of routing than a round trip, and it is not the part to improvise.
Overpacking for cold and underpacking for wet. Almost everyone does it once.
Assuming you will see everything from the ship. You will see a great deal. The bears, the interior and the real quiet are on the land days and on the small boats.