It's possible to unlock month 9 after month 10 starts to finish missions you missed, although I'm not sure on the specifics. I think you have to unlock that month's monthly medal ("Illuminate 7 maple leaves" for that month), but I'm not positive about that. I did this on my alt to finish up some missions since I started working on him too late to do everything.

However, it seems buggy. At one point, my alt went well beyond the requirements for a mission (and the game acknowledged that), but he didn't get all the points he should have and he couldn't make any further progress.