after the records are displayed in the report, I want to ... display this calculated data on the report in the month field.
Not possible. Once a report is generated, that's it. When February calculations are done, are the January ones done again? If so that's a monstrous amount of processing seeing as how there are more than 2400 for one registration for one year alone. The only way to avoid that is to store the calcs, which we usually advise not to do. I suspect that if there are 10 registrations and 204 per, it's still over 2K of calcs even if you store past data. I also think you'll need to use code, but not much idea of what that would look like at present.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.