HI, we cruised alaska on the mercury in september last year, 2 formal night and three smart casual are the norm. We found a large percentage did dress up, which was pleasing. If you do no want to drag your tux across the Rockies then you can hire through the celebrity website, formal menswear is the link and they deliver to your cabin on the first day and its yours until the last night. They provide full tux, shirt and bow tie - I found them good quality. The shoes are patent leather so you may wish to take your own. Have a great cruise, is awesome.
2 formal is the norm for a 7 night cruise and the last time i sailed with Celebrity a dark lounge suite was just fine,they do like the formal nights and can go over the top but they don't seem to worry to much what every one else wears?
You can hire a Tux onboard.
Also I am informed that from 1st August Celebrity are dropping the evenings were lounge suits are required, the two dress codes will Formal and Smart Casual .