The national day of Wales is St David's Day and is always observed on 1 March.
Saint David (known as Dewi Sant in the Welsh language) was a Celtic monk born towards the end of the fifth century and was Archbishop of Wales. He spread the word of Christianity across Wales, founding 11 churches across Wales and Brittany. His influence is shown by the number of churches dedicated to him in Wales.
It is always observed on 1 March, as the tradition is that he died on that day in 589 AD, though the tradition also says that he lived for over a 100 years, which would have been a mightily impressive age to get to in the sixth century.
While not recognised as a the national day of Wales until the 18th Century, the feast of St David dates back to 1120 AD, when David was canonised by Pope Callactus II. St David was recognised by some as the national patron saint during the period of Welsh resistance to the Normans.
St. David has had a few minor miracles associated with him, but the most famous story concerns the a large crowd who had gathered at the Synod of Llanddewi Brefi to witness the decision to make David an Archbishop. When David got up to speak, someone complained that due to the crowd, they wouldn't be able to hear him speak. At that point, the ground itself rose up, placing David above the crowd, so that everyone there could see and hear him. Needless to say, after that impressive feat, the decision to make David an Archbishop was taken.
In 2000, the National Assembly for Wales voted unanimously to make St. David's Day a public holiday and public support in wales has remained strong. Despite this, all moves so far to make St. David's Day a bank holiday have been rejected by the British Government.
This results in the situation where in the United Kingdom, Scotland and Northern Ireland have public holidays for ther patron saints, but England and Wales don't.
Every year parades are held in Wales to commemorate St. David. The largest of these is held in Cardiff.
On St. David's day, Welsh people may wear one or both of the national emblems of Wales on their lapel - the daffodil or the leek on this day. The tradition of wearing a leek is said to have arisen when a unit of Welsh soldiers were able to distinguish each other from troops of similarly attired English enemy soldiers by wearing leeks.
On 1 March the Empire State Building will be floodlit in the national colours of Wales - red, green and white.