First Sunday after Pentecost
The first Sunday after Pentecost, known as Whitsun in the United Kingdom, is Trinity Sunday. It is a day dedicated to understanding the Holy Trinity of God as Father, Son, and Holy Spirit, and traditionally involved personal reflection and prayer. Though not many folklore traditions are linked to it outside of religion, it was still a day of note in the historical calendar.
From the Roman invasion and occupation of Britain through to the late Middle-Ages, the concept of God as a trinity was one regularly reinforced and preached. Ancient Celtic gods were often triple deities and were embedded in the national consciousness, both in Ireland and Britain. By applying the thinking of three-in-one to the Holy Trinity, local customs could be adapted and beliefs changed over time to move away from pagan deities and instead towards the church.
And Jesus came and spake unto them, saying, All power is given unto me in heaven and in earth. Go ye therefore, and teach all nations, baptizing them in the name of the Father, and of the Son, and of the Holy Ghost: Teaching them to observe all things whatsoever I have commanded you: and, lo, I am with you always, even unto the end of the world. Amen.Matthew 28:18–20, KJV
Celtic gods such as Brigid and the Morrígan were each simultaneously a single goddess and three sisters, and so had differing attributes whilst remaining an individual entity. On Trinity Sunday, the Holy Trinity as a singular form—God—and three parts—the Father, the Son, and the Holy Ghost—was reinforced to parishioners using similar wording in order to apply the trinity concept already understood.
Trinity Sunday is still noted in some church calendars, particularly Catholicism, but it is no longer a national observance.