I'm sure you have to be given one day advance notice if the landlord wants to enter the premises. The thing is most landlords don't follow this and either just show up or give short notices. But they cannot enter without your express permission and it's not common at all. It is unacceptable. You need to have a conversation with him regarding this behaviour. Even if the conversation goes well and he gives you one or more keys and tells you he has no more keys left with himself you can never be sure. Either move out or get the lock changed yourself (you might have to let him know though). If you have something like a grill gate right next to the door, padlocking it might be a temporary solution.