This is based on the G1 lore, first seen in the My Little Pony comic #8 (G1) published in 1985, with a magical tree that can grant wishes, the unicorn Powder could summon snow - and the light in the opening that is closed in C3, can be a “Wishing Light”.

So the idea behind the Winter Wishday is taken from G1 but moved to the start of the winter season, TYT supports this as the episode after C3 shows winter activities like snowball fights. So the unicorns of Bridlewood when they still had magic gathered in the square in front of the tree to invoke snow and cold, freezing the temperature and increasing the humidity in the atmosphere. The ceremony seemed to start when three special unicorns stand in front of the opening and possibly ignite the magic in the tree during a ritual that has since been forgotten. These could be alicorns.

And since the unicorns use the same spell as the crystal ponies to transfer their magic to the crystals that store or recharge the magic, there is obviously a connection between Bridlewood which was likely a resort for the Canterlot unicorns, and the Crystal Empire ruled by Cadence who has her daughter, Flurry Heart.

The writers behind C3 know their stuff with reusing lore from older works, and integrating it into new lore in the G5. And it is possible that the Wishing Tree existed in the G4, as a mysterious tree was depicted on a glass window at the beginning of “The Last Problem” in S9, when Spike returned from the meeting between the abyssinians and the diamond dogs.