“May I remind you that payment is due at the beginning of the month?”
You babysit a couple's child. They are supposed to pay you at the beginning of each month, but they are always late every month. You're annoyed by it and angry at them. Now they are a week late on paying you. You say this because you want to tell them to pay while expressing your anger.
May I remind you that payment is due at the beginning of the month?
This is a forceful phrase that you use when someone isn't following the rules and you want to tell them what the rules are again. For example, teachers often use this:
May I remind you that all essays are to be turned in at the beginning of class.
This phrase looks very polite, but in practice people use it in speaking or writing when they are angry or annoyed by something.
When you're making a rule or re-stating a rule about when someone is supposed to pay a bill, use this phrase. As an example:
My car payment is due by the 15th.
You can also use other words that mean something similar to "payment" in a similar way:
When is the mortgage due?
Her rent was due last week.
This is a phrase that is often used in written documents, and in conversation as well.
Remember to use "at" and "of" with this phrase whenever you want to talk about the beginning of something.