Generally, a landlord cannot just turn up and demand to be let into your apartment as you are entitled to privacy and peace and quiet. The only time a landlord could enter your home legally without giving notice would be to deal with an emergency. However, there can be other situations in which a landlord could enter your apartment, provided they give “reasonable” notice (generally 24 hours or longer):
To show the property to potential renters or buyers
To make necessary repairs or provide services
If the entry is in accordance with your lease