Try to renew expiring lease or let it go month-to-month?

[Whitney Breedlove]

My tenant's lease will be up at the end of the summer and I'm debating if I should give them 30 day notice to sign a new lease term, extending the lease another year or two, or allow the lease to enter the month-to-month auto renewals? I have really good tenants and I don't want to lose them which I'm afraid may happen if I ask them to commit to another year or two but I also prefer my leases to begin in the summer when more people are moving. In the past, I've always received more interest in the property than I can even process when it's up for lease so vacancy has been close to 0% but it's always been on the market with an August move in date. I'm wondering how much of an impact there will be if I let them stay a few extra months, and then I'm potentially stuck with listing it in the winter or some other inconvenient time?