St. Piran was an Irishman who was said to have performed many miracles, such as raising soldiers slain in battle from the dead.
The Saint Piran Trust website says : According to legend, St Piran was born in Ireland in the 6th century. He was renowned for his miraculous deeds but a group of tribal kings grew afraid of his powers and jealous of his influence. They put a millstone around his neck and threw him off the top of a high cliff into the sea. As Piran fell, lightening and thunder raged, but as he reached the sea the storm ceased and the Irish watched St Piran float on the millstone towards the Cornish shore. After many days at sea, he safely landed on the beach that bears his name today – Perranporth. He built his chapel in what is today a large expanse of sand dunes and it is said that his first converts were a fox, a badger and a boar. The Cornish people flocked to see him as news of his teaching spread.
According to the Gospel of Wikipedia, there is little description of specific traditions associated with this day apart from the consumption of large amounts of alcohol and food during 'Perrantide', the week leading up to 5 March. The day following St Piran's Day was known by many as 'Mazey Day'. The phrase 'drunk as a perraner' was used in 19th century Cornwall to describe people who had consumed large quantities of alcohol.
It seems St Piran's Day is always held on 5th March each year. Tinners have a tradition that there are some secrets regarding the manufacture of tin which were communicated to their ancestors by St Piran and that, on that day, they stop work and treat the day as a holiday. It has only been since the late 19th century that Celtic revivalists managed to introduce a day of celebration to Cornwall. Saint Piran's Day has been celebrated throughout the County since the 1950's and now almost every Cornish community holds some sort of event to mark the day.
Thanks to St Piran there is a good excuse for the consumption of much alcohol !