YULE
On or around December 21, the winter solstice. Most often traced back to Norse celebrations of midwinter. For a more in-depth look at modern Heathen celebrations of Yule, I recommend A Guide to Celebrating the Twelve Days of Yule by Jenn Campus (pdf here! I think it's a legal source but please let me know if not). Even if you're not Heathen, I think that using Heathen sources can be really helpful for Yule in particular.
One thing to note in particular for Gaulish Polytheists: the eve of the Solstice is Mother's Night. This might be a good night to honor the Matronae and Matres, who are attested in both Gaul and Germania.
If you don't feel moved by any of the above, UPG has led me to a few other interpretations of the holiday. First, this is a time connected with the Wild Hunt. This could inspire some celebrations in its own right, but I also feel this would be an appropriate time to celebrate Artemis. There are many leaders of the Wild Hunt, and some place Artemis as their leader outright or syncretise her with other figures known to lead the Hunt like the Queen of Elphame or Holda. I also personally feel that the Summer Solstice has a strong association with Apollo. The idea of the siblings watching over opposite sides of the year is a beautiful image.
You may also feel drawn to stories of the sun disappearing or being reborn. Many of my own Yule celebrations have just been in honor of the Sun itself, without the intercession of a God feeling necessary. I have heard of Brittonic polytheists drawing a connection with Mabon ap Modron. If I remember correctly, he was rescued on the winter solstice after being kidnapped. The epithet of Apollo Maponus adds a layer of complexity to this and is endlessly fascinating to me.
On a practical note, Yule is a good time for doing prosperity magic. The traditional winter spices like ginger, cloves, and allspice have strong associations with prosperity and protection of the home. Many areas in the American South (and I'm sure elsewhere) have prescribed meals for the new year to ensure health and wellbeing.