I am on that sailing in May as well and can't find anything official. Some say its the 2nd and 7th night, others say one might be a port night and I have also heard there's a 3rd formal night but don't know which night it is. I don't plan on being "formal", just plan on a pair of slacks or leggings with a nice shirt. From what I understand, people don't get very dressed up for Alaska cruises. Its my first to Alaska, so guess I will see.