I've been trying to find some good sources on Latha na Caillich - something definitive on its origins and any customs associated with it - but I've not come up with much, other than passing mentions. On the one hand this is pretty much what I expected. On the other, it's a little frustrating.
I was hoping to get something down in writing before the day, but life got in the way of that plan...Seeing as I haven't got much, I thought I'd share the few resources I've found, in quote form, for anyone who's interested in hunting up some information on the day.
While it's referred to as Latha na Caillich(e), March 25th, it also coincides with Lady Day, the Feast of the Annunciation (when the archangel Gabriel appears to Mary to tell her that she is with child). Before 1600, March 25th was the official New Year in Scotland, and the 'official', fixed date for the Spring equinox. In England, Lady Day was the spring quarter day, which was marked at the beginning of February in Scotland, so the dating may have some English influence in terms of marking the start of spring proper. Carmichael notably makes no mention of Latha na Caillich, and instead has Mary or Bride as the agent of spring who finally defeats the cold.
It seems that much of the lore surrounding the season was meant to determine when it was best to start the sowing of certain crops that needed to avoid frost, or else marking when the danger of storms at sea had passed and everyone could be a little more confident of returning home with a good catch.
Seeing as Latha na Caillich comes from Scottish tradition, the sources are naturally skewed to Scotland, but I've found a few notes on Lady Day in Ireland as well, which I've included for comparison. Seeing as I've already posted Grant's notes on the Cailleach from Myth, Tradition and Story from Western Argyll, I won't post them again here, but they're worth a read for some context, since MacKenzie makes mention of it below.
The excerpts I've given are listed in chronological order, with Scottish sources and then Irish references to Lady Day. And so without further ado: