When you pay rent....

Before another month passes, set up a contract. Renting from family with no contract can lead to big problems.

Doesn't matter who you are renting from, you need a rental agreement. Both parties need to agree on a due date and a late date. Without one I would say the first of the month and late on the fifth or the move in day and five days later is late.

I would work out an agreement and stick to it.

You need a rental agreement. Then both parties need to stand by what they've signed. Rent should be due on the day someone moves in. If you move in on the 21st, rent should be considered due every 21st of the month after that unless both parties agree to prorate it.

If there's isn't a lease or any agreed upon legal paperwork, and since it is a relative, why not just ask the person what day of the month is their preference? A fair and family-friendly solution would be best. If it's someone that you don't care for or get along with, I'd say just do the best you can, then find somewhere else to live if there aren't any legal complications that are to be considered.

1st of the month.