When can a landlord enter the unit to show potential new renters?

I gave notice to quit the unit and now my landlord is showing up unannounced with new potential renters wanting to show the apartment to them. I do not let them in and tell the landlord that they must give me a minimum of 24 hrs notice. Under what circumstances is the landlord allowed to enter the unit? I am afraid to not be there if someone grabs something of my valuables when the landlord is not looking. I am also afraid the landlord may enter the unit when I am gone to work without my permission.