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. 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 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.
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. The Friendly Brothers of St Patrick, an Irish fraternity founded in about 1750, adopted green as its colour. 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.
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".