I "think" it's midnight of the 21st day before your cruise. Our cruise leaves on Sunday, January 15, 2023. My plan is to be online at 12:01 on Sunday, December 25, 2022 - I'll just stay up on Christmas Eve. It's easier for me to use the days, i.e., Sunday, instead of counting. I also have OCD, paranoia, a phobia of math and FOMO on the first check-in time 🙂 Jen