If it is only for Reporting purposes then @anikdas' solution will work seamlessly. But if you are trying to aggregate the numbers based on those weeks then you may have to take additional pain of converting Anaplan's calendar to Gregorian calendar. Beautiful article written by me friend @JaredDolich Link Below
Definitely have a look at the article @Misbah mentioned. It's a bit more than you need to identify the month but you can use the system modules. You'll want to be sure to include the logic for a leap year and decide how you will address the endpoints of your calendar. The last month of a 445 may end before the fiscal year (Gregorian) ends, for example December 27th. What will you do if someone enters December 28?
Thanks for the shout out @Misbah - you're amazing.