POSSIBLE:

 

1. The lengths of the months. I will concede to having the first two months of each quarter with 30 days, and the final month having 31. The final month must... have 31 days, thus allowing 25 December and 1 January to always be Sundays.

2. The leap year rule. I will concede, rather than my Base System, to use the rule proposed by Dr. Irv Bromberg in his Symmetry454 and Symmetry 010 Calendars. “It is a leap year only if the remainder of ( 52 × Year + 146 ) / 293 is less than 52.” I give reference to these pages for this rule - I DO NOT OWN THEM:- http://individual.utoronto.ca/kalendis/symmetry.htm- http://individual.utoronto.ca/kalendis/classic.htm

3. The name Intercalaris. The word intercalary is a common adjective meaning to do with being inserted into certain years (like our current 29 February), and the old Roman calendar, of which ours is an off-shoot, had a 22 or 23-day period in certain years sometimes called Intercalaris (although it was in February). So I have used that name for historical reasons. But I really don't care what you call it. It could be anything. (Like Mercedonius - another name the Romans used for their Intercalaris.) It could even be between September and October if you'd rather. But not between March and April, as that would muck around Easter dates for religious people; or between December and January, as that would do the same to Christmas. I just thought June and July was easier than September and October because it's mid-year rather than three-quarters. But it's not the be all and end all of my calendar.

 

IMPOSSIBLE:

 

1. The occurrence of 1 January and 25 December always on Sundays.

2. Every quarter year being equal

3. A constantly uninterrupted seven-day week - MOST IMPORTANTLY!!