No lease, no security deposit

My husband and I have been renting this apartment in Boston since Feb 15, 2004 from a management company. My husband dealt with a real estate agent. We paid first and last month's rent when we first moved in. Although we were supposed to receive a copy of the lease, we never got one. When we called the agent and asked for it, he promised he would get it to us but we did not hear anything after that, so we let it go. Although the rental application form (looks like a generic form) says that we are supposed to pay rent on the 1st of each month, the agent says since we moved in on the 15th, we should pay on the 15th of each month.

We have never missed a payment, every 15th we send a check to the management company. After a year, the management company did not cash the check for our Feb 2005 rent. The company continued to cash each monthly check after that.

We are now planning to move out on May 17. The landlord says we have to give 30 days notice and that we have to pay for the entire month, until the end of May. I told her that we've always paid on the 15th, not the 1st and she said it doesn't matter, we still have to pay for the whole month.

What is our best course of action?

The 17th of the month is not the 15th. If your rent was due on the 15th of the month, you should be out on the 14th of the final month (before your next rent is due). If you are there past that time, you need to pay for the next month's rent. You are likely no longer on a lease, but are under a month to month agreement. You rent by the month now. Either arrange to be out by the 14th of the month, turn in your keys, and have your post rental inspection by then, or pay for the next month. You should be able to prove that your rent ran from the 15th of the month by showing several years of check stubs or receipts to prove it wasn't due on the first. If you are out before the next rent is due, you should not have to pay past your last day in the unit.

As for the check that was never cashed, why didn't you bring that up then? You should have questioned management as to why they didn't cash your check. If the check had been lost, you could have stopped payment on it and issued a new check at that time. Since you didn't, you have been one month behind ever since. Your last month's rent will no doubt go to pay for that missed month. You will need to pay for the final month you are in the unit.