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.