As soon as the lease agreement ends, the tenant must move out. Your tenants have a responsibility to leave your property in the condition in which they found it. This is why we recommend that both tenants and landlords record the state of the accommodation when the tenant moves in through photographs and written statements (at least handover protocol signed by both parties).
You and the tenants should agree on how they will pass back the keys to you. Whether the tenant gives you (or an associate) the keys to the flat in person or leaves them in your mailbox is up to you, but we recommend that you are personally present when they hand back the flat so that you can immediately check the condition of your property.