The event is finalized only when finished. The sessions for the previous month(s) will not appear in the previous month's reporting. This only occurs when the event is finalized. The impact on the player ratings will remain as calculated during the previous month. The players will not see that impact until next month or the month when the event is ultimately finalized.
In the update, the sessions from the previous months will be processed as if they were from this month. It is of no consequence to your reporting or management of your events at the club.