Calendar 13 is an interactive version of the 13 month, 28 day calendar which places "day 0" on December 21 to mark the Winter Solstice. The leap day on nearly every fourth year occurs on a Calendar 13 "year 4" at the very end of the year (December 20). Calendar 13 follows the same leap day rule as the Gregorian calendar, making it simple to convert between the two (which you can do at the Calendar 13 website). This makes it easy for people to adopt the calendar in place of the standard Gregorian.
Another factor that may encourage adoption is that Calendar 13 has cast month names aside in favor of simple numbers.
Calendar 13 was created by Cody Jassman and is based on the Osiris diagram produced by John Michell.