The on-line booking system rolls to the next day at 8 am eastern.
It's actually available for at least 23 hours per day (under normal operating conditions). There is a regular maintenance window sometime in the wee hours of the morning.
Thank you! Wanting to book Aulani for June 1 and wanted to do it online the first day possible but wasn't sure what time. Sounds like I can do it at 8 a.m. that morning!