If your lease agreement does not allow the tenants the option of breaking the lease with a certain amount of notice, they are obligated to pay rent until you find a new tenant. They may also be liable for the costs associated with finding a new tenant, such as marketing. You have an affirmative obligation to mitigate your damages by taking reasonable steps to secure new tenants. It would be prudent to send them a letter explaining that they're on the hook for rent until a new tenant is found. This may delay their move-out date and minimize your losses. If they ignore you and move-out anyway, your best bet is a small claims action.
Disclaimer: You should discuss your situation with a local attorney, as the above statement is a general overview and does not take into account all of the factual considerations in your case. Nothing in this statement creates an attorney-client relationship.