By 'curved', I mean that these calendars try to fit the changing lengths of the Martian seasons. Vertum (northern spring) has 193 days, Sumwin (northern summer) has 179 days, Tumver (northern autumn) has 143 days, and Winsum (northern winter) has 154 days. These season lengths change by a couple minutes each year, but that's only a day difference over a thousand years, so these calendars won't be out of date anytime soon. By 'monthly perpetual', I mean that each month has a whole number of weeks in it. Intercalary days could be lumped together in short weeks called 'weks'. These funky naming ideas are Peter Kokh's, actually, but he wasn't completely serious about wek...
Off by a total of 4 days when you add one 'wek' at then end of Winsum. Although there are 3 month lengths, the two long one are the longest season and the short one is in the shortest season. So it can be rationalized, eh?
with 4 extra days to kick around, it's off by a total of 6 days -- not quite as accurate as the other versions.
spread the 5 extras amongst the short's, and you're only off by a total of 2 days! But of course you can't clump your short months together like you could in others. Who knows? Maybe that's a good thing.
Add the three extra days to Vertum, and they're only off by a total of 4 days.
also a total of 2 days off, if you don't mind a few 'weks' per year.
similarly 2 days off with a couple 'weks'.
Oh my, six through eleven, a bit more than I intended to. ...Pick your favorite and make it better!