The association of the seasonal feasts with life stages may be a Hearthsider practice that spread system-wide after the war of dissolution. Hearthside itself may reckon the feasts based on perihelion and aphelion (and the midpoints between), as I don't think a tidally locked planet has seasons… maybe I'm wrong. I'll have to check.

In any case, Hearthside has the briefest orbital period (shortest local year) of all the major bodies at Focus. This means the seasonal feasts occur more frequently relative to Yih, meaning Hearthsiders have lots of off days, contributing to the “Hearthsiders are lazy” stereotype.

As stated before, the seasonal feasts follow the local seasons, both of the planet and the hemisphere. Great Kindling is only a fixed date on Yih, where it's located at the midpoint between the fall and winter feasts in the Southern hemisphere. The date is arbitrary, chosen for being the midpoint between two less important feasts.

The feast celebrating First Meeting is actually reckoned according to the Terran Gregorian calendar. Since the Dewfall arrives on Earth around Christmas, that puts First Meeting on the pile of other winter holidays. While First Meeting becomes the preeminent Claravian holy day after First Contact, humans of whatever faith are encouraged to participate, and it becomes by far the most multicultural holiday among both species, though not in a watered down or commercialized way, pretty much anyone can get behind the idea that it's good we're not alone anymore.

There is also a feast celebrating the Theophany, though I'm not sure when it is or what it involves.

The four seasonal feasts, as well as the three daily liturgies, are inherited from primordial animism, and the Bright Way draws on this antiquity to legitimize their authenticity. “We were made to worship this way” is a common quote on the matter. The saying has a pleasing double meaning in English (and possibly also Hearthsider). “made to worship” means both “created to worship” and “instructed to worship”.