Last year I was able to participate in a book project and wrote some portions of Patrick of Ireland: His Life and Impact with Michael Haykin and Aaron Matherly (Amazon). This new book is the first of many in a new series: Early Church Fathers series (more info here). These books will be of value to any churchmen, pastor, or undergraduate seeking to gain familiarity on early church fathers, their literature, and personal piety. Patrick is not only a man of courage, exemplar in piety, but a great example to the modern person! Though scholarship is divided, he leaves us with two works (Confessions and Letter to Soldiers of Coroticus; original language). They are simple to read and would highly encourage anyone to read through them in order to gain familiarity with Patrick.
Leaving an indelible mark on the Christian church, especially mission endeavors, Patrick is born relatively around 390AD in Britain. Within his teenage years, Irish raiders invade and capture over a 1,000 British men and women, including Patrick (Conf. 1). He, thusly, served as a slave in Ireland for six years.
While in Ireland, Patrick was converted.
And there the Lord opened the sense of my unbelief that I might at last remember my sins and be converted with all my heart to the Lord my God, who had regard for my abjection, and mercy on my youth and ignorance, and watched over me before I knew Him, and before I was able to distinguish between good and evil, and guarded me, and comforted me as would a father his son. (Conf. 2)
Patrick, ultimately, attributes his captivity to his rebellion in his early years. Six years later, Patrick escapes from captivity and makes his way back to Britain. There, he was finally able to receive theological education, though he never really felt a sense of accomplishment in his reading and writing skills (Conf. 37).
The piety and spirituality of Patrick is intrinsically embedded in Trinitarian theology (Conf. 4). Despite the despair of Irish captivity, Patrick was moved by the Spirit (both physically and audibly in dreams) to return to Ireland. The impetus to come back to Ireland and remain there was the "binding" of the Spirit in Patrick's life.
But I am bound by the Spirit, who gives evidence against me if I do this, telling me that I shall be guilty; and I am afraid of losing the labour which I have begun—nay, not I, but Christ the Lord who bade me come here and stay with them for the rest of my life, if the Lord will, and will guard me from every evil way that I may not sin before Him. (Conf. 43)
So, St. Patrick's day is a day to remember his piety, example, and missionary accomplishments. St. Patrick's day celebrates missionary movement and spread of Christianity into land of Ireland.