Are landlords aloud to start moving items on to the property before they take posession of the house?

We have been here for almost two years and the landlord called on the 6th, one week after we paid rent and said we had to be out by January 1st. She went on to say her son was moving in because he was discharged from the army and had "an incurable disease". Today she called and left a message they would be here today to put up her grandkids swingset.