The Bonavian Civil calendar is a calendar reform proposed by Chris Carrier. Every month begins on a Sunday. Most months have exactly 4 weeks, but January, April, July and October have 5 weeks and also December in a leap year. The 5th week of December (when it occurs) is a leap week.
The leap years are those whose number when divided by 28 has a remainder of zero, five , 11, 16 or 22.
With these leap years the mean year would be 365.25 days, the same as for the Julian Calendar, but there is an exception: those years whose number is divisible by 28*32=896 with a remainder of zero are not leap years. This brings the mean year down to 365.2421875 days.
Epoch[]
An E-mail from Chris Carrier implies that year 1 began on Julian day number27 = Sunday 21 December 4714 BCproleptic Gregorian calendar. So year 6721 begins 240*1461-7 weeks later on Sunday 23 December 2007.