The contract will tell you the rent due date (for example, the 1st, 5th or 10th of the month).
Typically:
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the first rent payment for the period starting on the start rent date is due on or before that due date.
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In many cases, the first rent is paid before or at the beginning of the first month, so rent is paid in advance for the period that is starting.
Example:
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Start rent date: September 1
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Rent due date: every 1st of the month
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First rent for September is due on or before September 1.
Always follow the exact rule written in your Tenancy Agreement.