I learned from Wikipedia that the particular celebration of the Trinity came about due to the Arian heresy.
A Psalm of David, when he was in the wilderness of Judah.
O God, thou art my God; early will I seek thee: my soul thirsteth for thee, my flesh longeth for thee in a dry and thirsty land, where no water is; To see thy power and thy glory, so as I have seen thee in the sanctuary. Because thy lovingkindness is better than life, my lips shall praise thee. Thus will I bless thee while I live: I will lift up my hands in thy name.
My soul shall be satisfied as with marrow and fatness; and my mouth shall praise thee with joyful lips: When I remember thee upon my bed, and meditate on thee in the night watches. Because thou hast been my help, therefore in the shadow of thy wings will I rejoice. My soul followeth hard after thee: thy right hand upholdeth me.
But those that seek my soul, to destroy it, shall go into the lower parts of the earth. They shall fall by the sword: they shall be a portion for foxes. But the king shall rejoice in God; every one that sweareth by him shall glory: but the mouth of them that speak lies shall be stopped.