This is my way of organizing worship of Celtic deities based on the Roman or Greek deities associated with Them. After a couple generations of interaction with the conquering Romans, the Celtic speaking tribes who had the most contact with the Empire created their own versions of deities and rituals based on Roman religious practices. (Sulis Minvera is a great example.) If the tribes did that, then perhaps they also used the Roman Festival calendar to organize their offerings to different deities, like I do.
October 7th is the Roman celebration of Jupiter Fulgar (Jupiter “of the Daytime Lightning”) and Juno Curitis. Although Jupiter Tonans (“the Thunderer”) and Juno Regina fit very well for Taranus (as discussed in this post), who doesn’t want another scheduled day to honor the Gaulish sky storm God?
There’s another deity who came to mind. Lugh‘s spear of fire that never misses always seemed like lightning to me. With so many other Indo-European Gods throwing bolts of lightning, it’s odd that the Celtic people don’t appear to have such a deity. Most modern scholars consider Lug to be an ancient storm God who is associated with Odin in the Celto-Germanic culture discussed more here. (Old scholars thought He was a sun God.) The connection with agriculture is very common for Indo-European thunder storm Gods such as the most popular God in Iceland, Thor. The history of Lug is discussed more here. If you want a day for honoring Lugh, this date may make sense.