Father, Son and Holy Ghost. Usually taken to be a mystery, especially by those to lazy to think. There used to be a bishop wrote his episcopal letters so that his priests needn’t go to the trouble of making a sermon on this day.
Hooray for: one, three, philosophy
Decoration: shamrocks, or similar. Those Irish designs with the three interlocking parts.
Music: Te Deum (just), Firmly I believe and truly (only just), Eternal Father Strong to Save, and other trivial songs that mention Father, Son and Holy Spirit in separate verses.
Prayers: Sign of the Cross, Glory be, etc.