For the sake of simplicity and not confusing the issue of which month, you do not want PayAll payments that straddle the change of month. You also do not want PayAll payments that straddle statement periods. Keeping in mind that Citibank work around an extremely rubbery 3 day period, that sometimes can be 1 day and other times can be 5 days. It's 3 days ± 2 days effectively.
You do not want a statement date of 28th because to maximise the time you have the free money to earn interest with before you have to pay it back, you need to allow yourself a minimum of 5 days after the statement date for Citibank to make the payment to be guaranteed in all instances it will occur AFTER the statement for the previous period has been generated. This means for most months a scheduled PayAll payment date of 2nd of the month. A 2nd PayAll date will sometimes be executed by Citibank on the 4th of the month and sometimes on the 31st of the previous month depending on where the weekends and public holidays fall. If Citibank choose to execute it on 31st of the previous month, then it will fail and be rejected because you'll have already maxed out the previous month with the PayAll payments scheduled for that month proper. When a payment fails, that's it. There's no second chance. You're done until next month.
The worst case month is February in a non-leap year. You have to set-up assuming 28 days per month to cater for February. If your statement date is 20, then the first PayAll scheduled date is 25 and the second is 28, which for this purpose is the last day of the month. Do not go beyond this.
The end of the month is meaningless for maximising time on deposit before payback. The only thing that matters is when your statement date is. You want to get payments as close as possible to, but always AFTER your statement date. 5 days delay will always guarantee that no matter where the weekends and public holidays fall. Any less and you'll have a failed payment at around Good Friday for Easter and at the end of February. These are the worst case times of year you have to account for.
But having said all that, I may not know anything and you could have a better plan. I've only banked ~1.2mil points and earn $233/month in free interest each month doing this so far, so what do I know?