Saint Patrick's feast day, as a kind of national day, was already being celebrated by the Irish in Europe in the ninth and tenth centuries. In later times, he became more and more widely known as the patron of Ireland.^[24] Saint Patrick's feast day was finally placed on the universal liturgical calendar in the Catholic Church due to the influence of Waterford-born Franciscan scholar Luke Wadding^[25] in the early 1600s. Saint Patrick's Day thus became a holy day of obligation for Roman Catholics in Ireland. It is also a feast day in the Church of Ireland. The church calendar avoids the observance of saints' feasts during certain solemnities, moving the saint's day to a time outside those periods. St Patrick's Day is occasionally affected by this requirement, when 17 March falls during Holy Week. This happened in 1940, when Saint Patrick's Day was observed on 3 April to avoid it coinciding with Palm Sunday, and again in 2008, where it was officially observed on 14 March. St Patrick's Day will not fall within Holy Week again until 2160.^[26][27] However, the secular celebration is always held on 17 March.

In 1903, St Patrick's Day became an official public holiday in Ireland. This was thanks to the Bank Holiday (Ireland) Act 1903, an act of theUnited Kingdom Parliament introduced by Irish Member of Parliament James O'Mara.^[28] O'Mara later introduced the law that required that pubs and bars be closed on 17 March after drinking got out of hand, a provision that was repealed in the 1970s.

The first St Patrick's Day parade in Ireland was held in Waterford in 1903. The week of March 15-22, 1903 had been declared Irish Language Week by the Gaelic League and in Waterford they opted to have a procession on Sunday March 15. The Waterford trades unions from the Trades Hall decided to take part in this “St Patrick’s Day Procession”. ^[29]The procession comprised the Mayor and members of Waterford Corporation, the Trades Hall, the various trade unions and bands who included the Barrack St. Band and the Thomas Francis Meagher Band. This procession started from the premises of the Gaelic League in George's St. and finished in the Peoples Park, Waterford where the public were addressed by the Mayor and other dignitaries. ^[30] [31] On Tuesday, March 17 Waterford business premises were closed and marching bands paraded like they did two days previously.^[32] The Waterford Trades Hall had been emphatic that the National Holiday be observed and it appears that most business premises remained closed for March 17, including a lot of Public Houses. > ^[33]

The first St Patrick's Day parade in Dublin took place in 1931 and was reviewed by the then Minister of Defence Desmond Fitzgerald.^{[citation needed]}

In the mid-1990s the government of the Republic of Ireland began a campaign to use St Patrick's Day to showcase Ireland and its culture.^[34] The government set up a group called St Patrick's Festival, with the aims:

• To offer a national festival that ranks amongst all of the greatest celebration in the world
• To create energy and excitement throughout Ireland via innovation, creativity, grassroots involvement, and marketing activity
• To provide the opportunity and motivation for people of Irish descent (and those who sometimes wish they were Irish) to attend and join in the imaginative and expressive celebrations
• To project, internationally, an accurate image of Ireland as a creative, professional and sophisticated country with wide appeal.^[35]

Wearing of the green[edit]

On St Patrick's Day it is customary to wear shamrocks and/or green clothing or accessories (the "wearing of the green"). St Patrick is said to have used the shamrock, a three-leaved plant, to explain the Holy Trinity to the pagan Irish.^[13][14] This story first appears in writing in 1726, though it may be older.

In pagan Ireland, three was a significant number and the Irish had many triple deities, a fact that may have aided St Patrick in his evangelisationefforts.^[15][16] Patricia Monaghan says there is no evidence that the shamrock was sacred to the pagan Irish.^[15] However, Jack Santino speculates that it may have represented the regenerative powers of nature, and was recast in a Christian context – icons of St Patrick often depict the saint "with a cross in one hand and a sprig of shamrocks in the other".^[17] Roger Homan writes, "We can perhaps see St Patrick drawing upon the visual concept of the triskele when he uses the shamrock to explain the Trinity".^[18]

The colour green has been associated with Ireland since at least the 1640s, when the green harp flag was used by the Irish Catholic Confederation. Green ribbons and shamrocks have been worn on St Patrick's Day since at least the 1680s.^[19] The Friendly Brothers of St Patrick, an Irish fraternity founded in about 1750,^[20] adopted green as its colour.^[21] However, when the Order of St. Patrick—an Anglo-Irish chivalric order—was founded in 1783 it adopted blue as its colour, which led to blue being associated with St Patrick. During the 1790s, green would become associated with Irish nationalism, due to its use by the United Irishmen. This was a republican organisation—led mostly by Protestants but with many Catholic members—who launched a rebellion in 1798 against British rule. The phrase "wearing of the green" comes from a song of the same name, which laments United Irishmen supporters being persecuted for wearing green. Throughout the 19th and 20th centuries, the colour green and its association with St Patrick's Day grew.^[22]

The wearing of the 'St Patrick's Day Cross' was also a popular custom in Ireland until the early 20th century. These were a Celtic Christian cross made of paper that was "covered with silk or ribbon of different colours, and a bunch or rosette of green silk in the centre".^[23]

Much of what is known about Saint Patrick comes from the Declaration, which was allegedly written by Patrick himself. It is believed that he was born in Roman Britain in the fourth century, into a wealthy Romano-British family. His father was a deacon and his grandfather was a priest in the Christian church. According to the Declaration, at the age of sixteen, he was kidnapped by Irish raiders and taken as a slave to Gaelic Ireland.^[12] It says that he spent six years there working as a shepherd and that during this time he "found God". The Declaration says that God told Patrick to flee to the coast, where a ship would be waiting to take him home. After making his way home, Patrick went on to become a priest.

According to tradition, Patrick returned to Ireland to convert the pagan Irish to Christianity. The Declaration says that he spent many years evangelising in the northern half of Ireland and converted "thousands". Tradition holds that he died on 17 March and was buried atDownpatrick. Over the following centuries, many legends grew up around Patrick and he became Ireland's foremost saint.