The St Patrick's Day Parade and everything Irish are celebrated on March 17th, his religious feast day. While most Major cities with an Irish population will have a St Patrick's day Parade, that was not the origin of the celebration. Since his death in the 5th century it has been a religious holiday traditionally used to pray for missionaries around the world, followed by an afternoon of celebrating.
The first parade took place in 1762 in the United States. Irish soldiers who were in the English army marched in a parade on March 17th. As the number of Irish immigrants began to grow in the US, they banded together and formed clubs like the Hibernian Association and Friendly Sons of St Patrick.
During the potato blight of 1845 large numbers of immigrants from Ireland came to America. To help them assimilate the new immigrants lived in tightly packed communities. Local civic leadership at the time organized the citizens and the 'green machine' or Irish voting public had the ability to influence elections. To get the attention of this voting block politicians started to become associated with and march in the parades and festivities. In 1948 President Truman became the first president to attend the parade event.