The 28/293 calendar is a 13-month calendar invented by Victor Engel, where each month has 28 days not counting a special day that occurs exactly once every 294 days or 42 weeks. Every day, including any special day, belongs to a standard 7-day week.
This results in a 293-year cycle, that has 364 special days, so is equal to 294 periods of 364 days. The mean year is therefore about 365.24232 days.
Days and months are counted from 0 rather than 1.
Year 0 begins on 21 December 2012 and its first month (Month 0 Year 0) ends with a special day.
The same author invented a similar calendar called the 43/450 Calendar.