Why not talk to the charter opperator first. It really depends on where he's located as to what the fishing will be like. Its like asking what week is best to hunt deer in the L48... Way to many variables. Its a heck of a long ways from Sitka to Dillingham, and a heck of a lot of salmon fishing between the two.
Red/sockeye "fishing" is not really fishing, its snagging. May-August depending on where. If the fish are in, you should do ok.
Kings run from May-August depending on where you fish,
Silvers run from August - November depending again...
Pinks/chums, fun to fish for, but crap to eat. July-August
Word of caution... King fishing SUCKS pretty much state wide and has for the last couple years, don't think next year will be any better. Don't book a trip for kings and expect it to be "good". There was a lot of guys burnt last year by outfitters/charters who booked kings but couldn't fish for them because the season was closed by emergency order. Not to really fault the guides, but they should have warned their perspective clients that the king fishing has sucked and would likey suck again... It was no secret that the king fishing was going to be bad last year.
Silver fishing has also sucked the last couple years, and will likely again next year.
The only fishing that has been consistent and will likely be is for red/sockeye. If you hit it right you can fill the boat, miss it by a week or 2 and you're eating at Long John Silvers.