To book excursions, Palo, and spa services it is midnight. For online check-in and choosing your port arrival time I believe it is 5 a.m. (although I have heard as early as 3 a.m. and as late as 7 a.m.). I did our last online check-in I think it was around 5 or 6 a.m. and was still able to snag an 11:00 a.m. port arrival time.