So, I worked up a Calendar for use in my Saga, and I thought it might be of some use to a few of you, therefore I will share.
It's pretty simple. One year per page, Seasons are highlighted in colors, Full and New Moons are displayed via outlined and filled circles and Sundays are highlighted as well. Since it's based on the Julian calendar, the Seasons are in odd places to the modern eye, but they're correct (as far as timeanddate.com can tell me). It covers the years between 1196 and 1245, which is about as far as I can optimistically project my Saga will last.
As usual, if you see any errors/bugs, please let me know.
The solstices and equinoxes are the first days of the seasons (by the definitions of the seasons). In Emirikol's doc, we get green spring equinoxes, red summer solstices, orange autumn equinoxes, and grey winter solstices. Like a medieval box of Lucky Charms.
But some cultures consider the solstice/equinox the start of the season, others consider it the midpoint of the season. What does the Order consider it to be? (That's always confused me running Ars sagas.) Reference please!
I thought about adding the Cross-quarter days, but laziness prevailed. That, and the fact that the only Hermetically important cross-quater day that I know of is Walpurgis Night, which is the traditional start of tribunals in the Rhine. Since my Saga is in Provencal... shrug. On the rare occasion that I'll set a story on one of the other cross-quarter days, I'll just do some quick math and make a note.
I don't stretch the logic of this too far as it's basically a simple justification for fiat, but my approach has been to say that since year-long spells terminate following equinoxes/solstices, these mark the end of one season and the beginning of a new one, rather than marking the middle. If there's a formal Hermetic reference, I don't know of it.
Interesting - in real life I take it as a given that seasons start on the solstices and equinoxes (and therefore have no trouble extending that belief to Mythic Europe). But you're definitely correct that different cultures begin their seasons at different points.
In short, I can't find any reference in ArM5 about the specific starting points of seasons. Bede's observation about the expiration of Year Duration spells resonates with me, but I admit that's indirect reasoning.
Is there an easy way it could be extended back as far as around 700 AD (or maybe 45 BC)? I tend to work on my own version of Mythic History going back a ways, and such a resource would be very helpful.