The “Roman” Catholic church is also celebrating the Feast of the Holy Trinity, today, although it is separate from Pentecost which was last week.
Orthodox ping!
“Glory to the Father and to the Son and to the Holy Spirit, the triple and single light of the one Godhead, that exists singly in Trinity and is divided without division. For the one God is Trinity, whose glory the heavens declare, while earth proclaims his dominion, the sea His might and every physical and immaterial creature his greatness. For to Him belongs all glory, honour, might, greatness and magnificence, now and for ever, and to the ages of ages.” Liturgy of +James
I happened last night to be reading a history of World War I and the fighting on the eastern front. One of the Tsarist generals refused to mount an attack on the Austrians because it was Holy Trinity Day.