Rules for Lanlords and tenants vary greatly from state to state, and sometimes even from County or City.

Landlords have a right to show the place, and it is usually outlined in the lease. Your landlord should have given you just one notice of what days the unit was going to be showed. Also, notice to entry is not the same for all states. Some have no statute regarding entry. others differ in the time required prior to entry. I have provided a list of each state with the corresponding amount of notice required. You should always check at the municipal and county lever, but the table below should be a good basic guide.

State  Amount of Notice Required for Entry
Alabama No statute
Alaska 24 hours
Arizona 2 Days
Arkansas No statute
California 24 Hours
Colorado No statute
Connecticut Reasonable Notice
Delaware 2 Days
District of Columbia No statute
Florida 12 Hours
Georgia No statute
Hawaii 2 Days
Idaho No statute
Illinois No statute
Indiana No statute
Iowa 24 hours
Kansas Reasonable Notice
Kentucky 2 Days
Louisiana No statute
Maine 24 hours
Maryland No statute
Massachusetts No statute
Michigan No statute
Minnesota Reasonable Notice
Mississippi No statute
Missouri No statute
Montana 24 hours
Nebraska 1 Day
Nevada 24 hours
New Hampshire No Requirement
New Jersey No statute
New Mexico 24 hours
New York No statute
North Carolina No statute
North Dakota Reasonable Notice
Ohio 24 hours
Oklahoma 1 Day
Oregon 24 hours
Pennsylvania No statute
Rhode Island 2 Days
South Carolina 24 hours
South Dakota No statute
Tennessee No Requirement
Texas No statute
Utah No Requirement
Vermont 48 hours
Virginia 24 hours
Washington 2 Days
West Virginia No statute
Wisconsin Reasonable Notice
Wyoming No statute