Yulestice

Yulestice takes place around what is the Winter Solstice in the Northern Hemisphere, while it is the Summer Solstice in the Southern Hemisphere. It is celebrated as a time for togetherness, particularly with family, and to feast on the remains of the earliest parts of the harvest that won't last through the winter. Although most cultures have some kind of celebration to mark the solstices and equinoxes, Yulestice in particular originated in the northern parts of Mirrarium, where the winter is particularly harsh and darkness abounds with long nights. (In some areas north of the Sunless Circle, the sun doesn't rise for days at a time.) It is technically the start of winter, but due to the cold climate of the northern regions, it often feels more like midwinter, and is the longest night of the year. This is why there are traditionally warm fires, huddled up with friends and family. Many candles are lit, and food and drink is used to further keep warm.

History

In the northernmost regions, a fire is lit the night before the solstice itself. Tradition dictates that it is unallowed to go out until the day after the solstice. It is said that if it were to go out, those unprotected by its light and warmth will be stolen away by the Lord Ice, an Ice Giant who rides up on a sleigh pulled by Wendingos. Offerings of small cakes and milk are typically laid out in hopes that Lord Ice will eat his fill and ignore the house even if the fire goes out, although this may be a syncretisation of two separate traditions.
The families inside share a meal, usually of lamb, calf, or whole chicken although in towns with more of a tradition of hunting than farming this will often be venison or game bird instead. Fresh warm sweetbreads are served as side dishes.
There is a tradition of gift-giving, athough this was likely influenced by a more southern practice also added to Yulestice celebrations. In their celebrations, Bishop Winter, a figure similar to Lord Ice but far more benevolent visits homes to leave sweatbreads, cakes, and small items of entertainment value such as books, toys, and games. This derives from ancient traditions of struggle and frugality, where a family would typically not have many resources to dedicate to luxury. Because the northern regions were even harsher, "luxury" to them was a particularly warm fire and good food.
It is uknown just how linked Bishop Winter and Lord Ice are, whether the myths influenced one another. Bishop Winter rides a chariot (rather than a sleigh) pulled by unicorns, but scholars are unconvinced that this is anything more than a coincidence. The story of the figure visiting every family is a more enticing link. Some people say that they are really the same person and whether he comes benevolently or malevolently depends on if the family has obeyed the traditions correctly. Researchers don't believe that this is an original tale but rather a later attempt to link the two traditions, and note that some stories keep their respective roles in reward/punishment but maintain two separate figures. This kind of duality shows up in some religions with good versus evil deities, or merciful and punishing angels.
Although Bishop Winter has always been called a Bishop, the church or deity he is supposedly a Bishop of varies by the teller, typically inserting him into whatever religion the family follows.

Execution

The modern holiday is mostly "let's have a party when it's dark and cold because that tends to be a stressful time for most people" which is really as good a reason as any, and pretty much the original point of the celebration. The tradition of "great feast" has been kept, although most celebrations - of any holiday in any culture - have that. One piece of the tradition of Yulestice that has been kept by the more modern practice is that of lights. Decoration with candles and other lightsources is fairly widespread. Some alchemists even take a break from their grumpy no-nonsense or joy approach to life and use their skills to make Alchemists' Lights, which are candle-like wax sticks that glow through an alchemical reaction, making them far safer for decoration than a bunch of open flames. Alchemists lights are inexpensive and easy to make even by new apprentices and so are given out fairly freely during Yulestice.
Decorations of various kinds of plants are also used. This is done to remind everyone of the beauty of the natural cycle. Although many plants seem to be dead in winter, they miraculously come back to life in spring. Keeping these plants around keeps that hope alive.