Every other holiday. If the holiday falls on a day that would normally be your day off, you still work the holiday (e.g. if the holiday fall on your weekend off, you still have to work even though it's not your weekend). Holidays are: New Years Day, Easter, Memorial Day, Independence Day, Labor Day, Thanksgiving, Christmas. Since it's an odd number, the holidays you work are opposite every year. This seems like the most fair way to do it.