Comments / Suggestions!
I think if you're changing the activity into make the end of the month the cut off for activity that counts, you should maybe make that the last day of the check-in, and just announce it sooner?
Or announce it sooner and still keep it open a day or two after, but having 4 days (December 28th - 31st) to play catch up if you've had a hectic month and neglected a character or two after the post goes up'll be kind of hectic.
And I've been burned by games that did it this way in the past. Just a thought!