changing commission rate date of arrival or date of booking?

I will be changing commissions as of May 1 to 22%. Is this the date of booking or the date of arrival??

That would generally refer to the booking creation date, ergo any bookings created in OwnerRez after that date will attribute to that owner. Many PM settings for bookings, lock in at the time of booking creation. These settings will typically not retroactively update in many cases, however you can take actions such as running a batch update to force the system to update older bookings to current PM settings.

Batch update can be found via PM > Batch Updates

~Caleb