Another commonality among "Celtic nations" is the importance of their patron saints, and their feast days. These have become something of national holidays in their respective regions, in some cases, officially.
Prior to Christianization, Celtic peoples celebrated traditional pagan holidays, which tended to fall on solstices and equinoxes, to mark the transition of seasons. These took slightly different forms and different names, although there were commonalities; for example, there was a common late-summer harvest festival that commemorated the god Lugus. If any of these are celebrated nowadays, it's usually meant as a revival of old customs, although the tradition stayed in Gaelic-speaking areas until relatively late.
After Christianization, identification with patron saints became common, and like many other places in the Christian world, the patron saint's feast day became important. Of note are:
Wales: St. David's Day, March 1.
Cornwall: St. Piran's Day, March 5.
Ireland: St. Patrick's Day, March 17.
Galicia: St. James' Day ("Día Nacional de Galicia"), July 25.
Scotland: St. Andrew's Day, November 30.
Mann has St. Maughold, but his day isn't celebrated significantly AFAIK. Brittany has St. Anne, but rather than having a celebration for her feast day, they instead have something called
Tro Breizh that involves a pilgrimage to cites associated with other Breton saints.
Another fun fact is that the different regions sent saints to each other. The patron saints of Cornwall and Mann were Irish, many of the Breton saints were Welsh, and St. Patrick was probably a Briton.