The common year of the calendar consists of 364 days, which are divided into four quarters of 91 days each. The first month in each quarter consists of 35 days, and the next two months of 28 days each. Months and weeks always begin on the same day of the week. Every month in the Original Bonavian Calendar looks like this:

Five times in 28 years, specifically in years which, after division by 28, leave remainders of 0, 5, 11, 16, or 22, a leap week is added. This addition of 35 days every 28 years brings the average length of the year to 365.25 days. In years which are divisible by 896, however, leap week is cancelled. This reduces the average year length to 365+(31/128) days, a value an order of magnitude more accurate than the Gregorian calendar's 365.2425.

Leap-week calendars on the Bonavian model are a reasonable compromise between those who wish to preserve the seven-day week, which the Bonavian Calendar does, and those who wish a simple calendar in which each month and quarter can be easily memorized.

There are now three versions of the Bonavian Calendar; the first is the Original Bonavian Calendar, written by me in 1970 back when I was 11. In my case, the big annual event was the last day of school, which at all schools, whether they be public, religious, or private, in Ventura County, California, where and when I grew up, fell on the second Friday in June. This was the logical New Year day.

A name for the calendar wasn't settled on until about 1975. I had given it a number of names, "Leap Week Calendar" and "Arcturian Calendar" (after Arcturus, prominent right after sunset in the North Temperate Zone at Original Bonavian New Year) but eventually I decided to name it the "Bonavian Calendar" after my high school, St. Bonaventure H.S. in Ventura, California.

My second calendar is named the Bonavian Civil Calendar. It features quarters of 35+28+28=91 days, however, this time, the year, quarter, month and week all begin on Sunday and end on Saturday, instead of from Friday to Thursday as in the Original Bonavian Calendar. This looks more familiar than the Original Bonavian system to the lay person in the Western world who is used to the week beginning on Sunday or Monday.

In addition, leap week was moved from mid-August to the last week of the year in December, as it makes computing the number of days between dates easier if the intercalary days are at the end of the year, as they are in the Islamic, Persian, French Republican, both of the Egyptian, and several other calendars.

The third, Bonavian Leap-Month Calendar, was not invented until 1993. It was inspired by that wonderful book by Evitiar Zerubavel, The Seven Day Circle. The Bonavian Leap-Month Calendar was given the same synchronization as the Bonavian Civil Calendar, and although it featured 91 day quarters, it divides them as The World Calendar: 31+30+30 with years and quarters beginning on Sunday and months beginning on Sunday, Wednesday, and Friday in the three months of the quarter.

In years divisible by 28, a thirteenth month of 35 days duration is added, named Mercedonius, which, since we are still using Roman names, is logical because that was the 13th month name the Romans were using when they still had leap months pre-Caesar. The 35 day leap month is shortened to 28 days in years divisible by 896 to fix the average year length at 365+(31/128) days.