Ancient calendars based on a lunar cycle of 29 or 30 days resulted in a year 11 days too short, so many institutions added a leap month whenever deemed necessary.
Julius Caesar changed the ancient calendars; decreeing that a year has 365 days, and that every fourth year (starting in 45 B.C.) is a leap year with an extra day.
April 2021 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 _2_3 24 25 26 27 28 29 30Caesar's calendar resulted in three leap years too many every 385 years, so Pope Gregory XIII (with help from an astronomer) introduced a new calendar in 1,582 A.D. that called for a leap day in every year that is exactly divisible by four. To further make things work out correctly, the Pope decreed an exception to this rule: leap years ending in 00 (including 2000) must be evenly divisible by 400 to qualify for the extra day. The Gregorian calendar has remained universal in Western countries and Japan, Egypt and China.
April 2021 S M Tu W Th F S 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28All of the previous calendars had varying numbers of days in each month. While not especially difficult to compute the Gregorian calendar, there are fourteen different versions depending on the day of the week the calendar begins on and the existance of a leap year. The Alternative calendar proposed by Daemous Riel in 1,994 A.D. is a calendar designed for simplicity. The adoption of the Alternative calendar in modern life is unlikely because of the existance of various scheduling devices and humanity's need for variety, e.g.- the Gregorian calendar's ideosyncracies. However, a 13 month lunar-bound 28 day regular cycle is a clean and attractive approach.
The Alternative calendar consists of thirteen months. Each month consists of 28 days. A month has four weeks with seven days in each week. The numerical day of the month is not necessary because the first of the month is always the first Sunday, regardless of the month in question. Similarly, the eighth is always the second Sunday of the month and the fourth Saturday is always the twenty-eighth day of the month. Every year has a New Year's Day which is a single day outside of the other thirteen months. This day is not referred to as a day of the week. This day is typically 24 hours, but adjustments (e.g.- "leap seconds") may occur on this day. Every twenty-eight years, the New Year's celebration is eight days long instead of one day long. New Year's Day is officially the last day of the eight day long celebration. Again, this celebration is not part of a calendar month, however in this case the seven days are sometimes referred to as the Leap Month (perhaps in reference to the ancient calendars) and they are referred to with days of a week. In exceptionally rare times of calendar adjustment, a whole day may be subtracted from the 28 day New Year celebration reducing the New Year to only seven days.