The Markelsian calendar is a 13-month calendar proposed in 2000 by Minot State University psychology professor Dr. Paul Markel, based on the thirteen Zodiac constellations. The calendar has twelve 28-day months, and a thirteenth month of 29 days called Ophiucus. The first and twenty-eighth days of each month are always Sunday and Saturday, respectively. Every leap year an additional day, called Julius, is observed immediately after 29 Ophiucus; this day does not belong to any month, but is referred to as the Corrigendum. Markel says of this feature of his calendar: