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I'm Lucy, and I'd be happy to answer your questions today.
If your lease ends mid-month, meaning that you're only responsible for rent for 17 days in May, then the last month's rent you paid would be applied to the last 30 days of your stay. Massachusetts law is very favorable to the tenant, and he cannot evict you for non-payment of rent he's holding in a bank account for your benefit (which is what he's supposed to do). He'd be allowed to insist that you pay April's rent in full if the lease ended at the end of May and you were just planning to leave early, but that's not the situation you've described.
Your landlord also has to pay you interest on your last month's rent when you leave.
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