Annunciation. It arrives almost unseasonably, or at least surprisingly. As a homily yesterday observed, it starts the cycle of events that will culminate in next year's Passion. (Although arguably one shouldn't insulate it from the coming events, like two books being read at the same time. It also contextualizes the present moment in the cycle of events.)
And perhaps the unseasonable or surprising arrival carries forward some central attribute of the event. Long habits create rhythms of thought, but the new, by its essence, arrives according to its new logic. Christianity didn't arrive within Judaism according to the logic of the Jewish liturgy or seasons. Not a rejection of the old, but simply the next thing.
One almanac records that this is the beginning of the old Roman hilaria, the first festival after the spring equinox.
φοσ ιλαρον...