Ultra Beasts

Ultra Beasts is a term describing the seven powerful Pokemon each with a stat total of 570, all of which were introduced in Generation VII. They were first implemented into Project: Pokemon in the 1.9.0 patch.

Wish said on Discord:

I actually don't know when the Pokemon changes. It's definitely not midnight. I think it is around 5 PM that the Pokemon changes with the new script. For example, Saturday at 5 PM --> Sunday Pokemon, Sunday at 5 PM --> Monday Pokemon. etc. Time may not be accurate...

Keep in mind that the time you have is what wish_z put for Project: Pokemon (US)