Memorialize data in report

Hello,

I have following issue with developing a solution for my internal customer.

We have table with some events. We have date of event and, let's say state: open or closed.

My customer wants to see table with number of events and percentage of closed events by month.

BUT: after monthend they want to "memorialize" data. So, even if they will close some issue after one month, but at the end of month they had 99% of closed events, they want to still see 99%. But current month should be updated during the month. The only one solution that came to my mind is to create some aggregates directly in database, but I would like to avoid this, because if I change any logic in report I need to also go into sql query. I am wondering if is there any solution that I can "freeze" the data in DAX metric? Metric that will recalculate results only in current month, but no recalculate results from past months. Any ideas wise people?