Something which has always confused me with Ars Magica is that they never seem to define the dates when seasons start and end. This becomes relevant because story events sometimes happen on a Solstice or Equinox, traditionally high/low points of various magical cycles.
The modern definition seems to put spring solstice about three weeks into the spring season, but I've only ever seen Ars played with seasons where each solstice/equinox is between the seasons (so you can potentially have 20 continuous days to adventure without affecting your lab total, 10 days from each season).
I suspect that the Hermetic calendar uses Astronomical rather than Meteorological (Roman) reckoning, so Spring season would start on the Spring Equinox etc., but it would be nice to know if there is a canonical reference for this.