This may seem like a stupid question, and it probably is...
When, exactly, do the various hermetic seasons start? Is it on the Solstice/Equinox like I've been assuming, or do they fall on other dates...?
Thanks for your time.
TMRE pg. 48
"The Horoscope is prepared at the start of the season, on the day of the equinox or solstice".
It would appear so.
In my Saga and gaming group, we've done the following, to keep things simple:
We follow the seasonal change completely, and although we do use the dates on the regular calendar, we do not record the change of year on dec. 31st. We acknowledge the fact than the church and the mundanes do this, and speak to them in their terms, But for our own purpuses and for our chronicle (where we record all important events and loosely write up the stories as they happen, all in character of course).
Spring season starts on march 21st, this is also the start of the "Hermetic Year" if you will.
Summer season starts on june 21st
Autum season starts on september 21st
Winter starts on december 21st, at the end of this - just before the new Hermetic year starts - is where we apply Aging, cast Aegis. This is for simplicity, and for the symbolism. At the end of Winter, we do the bookkeeping, before the cyclic year starts all over again. And we don't have to remember to do anything else on the other quarterly dates.
These dates are also the venues for our regular council meetings, and where we upate the vis lists, since all our sources tend to yield something on an equinox/solstice. These meetings are usually nothing more than planning the following season's work for the magi, allocation of resources etc. It is also - curiously enough - very often the time where, after all important issues are dealt with, a servant runs into the chamber, and calls out: "The caravan is being attacked", or "There is an angry priest in the couryard" or something...
So in conclusion, when we write the heading for the chronicle, we'll write "Winter 1249" meaning the time from dec 21st 1249 until march 21st 1250. Of course, if something happens in the middle of a season - this happens often, making the Magi have to decide whether to go on adventure and risk spoiling their lab project, or stay inside in lab rat mode - we naturally use the regular dates, but refrain from using the year, to avoid confusion, because we change the year almost 3 months later than others.
I just start with winter (Jan; Feb, March), spring (Apr, May, June),...
It is against the rules, but it is easy.
I think the concept is complicated anyway, because not all characters are born on a solstice, so we get parts of seasons anyway...
A SG could decide that everyone is born on such a day (what a magical coincidence)...
The exact day a character is born is meaningless. Aging represents more the experience that the character has gained up to that moment and the bodily growth (and decrepitude) it has accumuloated. It can be argued that you roll for aging when your experience and body fatigue reach 35, not when 35 happens to be your actual, counted day by day age. For the day being born,
Autumn used ot be quite common for peasants (those rather boring winter months....). Summer is more common for undesired peasant babies (those summer pastures...) if you care for such details. I might be way out of the actual mark here, though, since I am in no way a demographer. Read it somewhere, thoguh.
Around here we go for an equinox/solstice seasons change calendar. We do not tend to take such exact day recording into account in most adventures, though, so no biggie. IIRC in the canon text it says that the hermetic calendar starts/ends in the winter solstice, whern the Aegis ritual is recast throughout the OoH, so it would seem to be a sensible date for starting/ending stuff 
Cheers,
Xavi
Weird they all do it the same day. I'd keep the date secret so my enemies don't know when to strike
The Aegis description in 5th ed. implies that the ritual might not last the whole year through if cast at other dates. (Though summer solstice ought to be as safe as winter, I think.)
The reason for that implication is that it has a duration of "year", which is until the 4th solstice/equinox after casting. So if you cast it the day before the spring equinox, it only lasts 9 months as it still expires on the Winter Solstice (as it will still hit spring/summer/autumn/winter).
If you cast on the Solstice or Equinox, it'll last a full year. Any other day and it will be less than a full year.
Also, doesn't ArM5 imply that magic auras are sometimes higher in winter, which is why it's a common time to cast the Aegis?
There's a throwaway line about auras sometimes being stronger on the winter solstice. AFAIK, there's nothing in the rules. I'm inclined to think its a reference to the Equinoxes and Solstices being major astrological deals and big pagan holidays. So that would likely apply to all 4 seasonal solar specials.