Based on the information you've provided (i.e. the word "resigning"), it sounds like you don't have a lease after August 1 as it stands. Unless written in the lease, there is not a requirement to renew a lease at the end of a term. Your landlord is saying they will renew your lease if you tell them by April 1 that you want to do that. If you're not going to renew, they presumably start looking for new tenants and are giving themselves a ton of time to do that. It definitely puts you in a tough situation, but from what you've said I'm not aware of anything that makes their request illegal.