Which nights are formal and informal on 7 day cruise?

[miami mama]

Does anyone know exactly which nights are designated as formal and informal on a 7 day cruise? I am currently trying to plan what to bring for my DH and me (especially since there are many formal selections in the stores for the holidays). Our Alaska trip on the Summit next summer has a late night or two in port and we will probably take advantage of the local cuisine if nothing special is planned on board. Thanks for the assist.

[cruisinelsewhere]

While I have not cruised Alaska, but have cruised X extensively, generally speaking, 1st, 4th and last nt cas, 2nd nt and 6th nt formal, 3rd & 5th nt informal, spoke to CC rep today about a Caribbean cruise in a few weeks and that remains the same... have a great time!