Does the tenant have a lease? If so, you must follow the terms of the ease. the sale does not invalidate the contract with thee tenant. If it is a month to month lease, and there is no contract, you must provide one full rental period of prior notice to vacate.
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Unless otherwise agreed to between you and the tenant, you take the property subject to whatever lease the tenant has. If it's a month-to-month lease, you must give notice to vacate on or before the last day of the month indicating that they must vacate by the end of the following month. A written lease may make other provisions as to termination and vacating the property. As an example, let's take your case--if the tenant has a month-to-month lease, you would have had to give notice on July 31 that you wanted them out by the end of August. If you want them out by the end of September, you need to give notice by the end of August. If they don't leave then, you have a bigger problem. Come back with more questions.