Tenant Did Not Leave After Lease Ended

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First property has been a fun one! Learning a lot. 

This time, a tenant is purchasing a house, and their closing date was postponed. Therefore, they decided to stay. Original lease end date was Sept 9. They say they'll be out by Sept 14. I have new tenants moving in Sept 15. I gave myself a week for repairs/cleaning/painting.

Now, my concern is: What if closing doesn't happen on the 14th.

I've read up on the laws around eviction, hold-over, and filed for eviction, but that won't happen until after the new tenants are supposed to move in.

What are my responsibilities to the new tenants if the property is not available on the 15th? 

Thanks in advance!