Hi, this issue comes up quite often on the Community.
This thread should explain: Changing the rent collection date
The short answer is that it doesn’t matter what day the rent is paid as long as it is paid before the due day. If you want to pay on the 21st, you can pay on the 21st – as long as you are paying in advance (i.e. for the upcoming period) then this is fine. There’s usually no need to put that in writing or make a change to the contract.