March 5 is St. Piran’s Day (Cornish: Gool Peran) and is the national day of of Cornwall, named after the patron saint of Cornwall and tin miners. According to tradition, some of the secrets about the manufacture of tin were communicated by St. Piran. I find no traditional rituals other than the consumption of alcohol.
The modern observance of St Piran’s Day as a national symbol of the people of Cornwall started in the late 19th and early 20th century when Celtic Revivalists sought to provide the people of Cornwall with a national day similar to those observed in other nations.
Since the 1950s, the celebration has become increasingly observed and since the start of the 21st century almost every Cornish community holds some sort of celebration to mark the event. St. Piran’s Flag is also seen flying throughout Cornwall on this day.
Summarized from Wikipedia.