Latha na Sàbaid is generally used by Protestants. DiDòmhnaich by Catholics.
So is Sàbaid for Christian Celts and DiDòmhnaich for pagan Celts?
Apparently Sunday can be either, "Là na Sàbaid" (lah nuh SAH-bidj) or "Didòmhnaich" (ji-DAWV-nuch)
Is it also ok to say, "they will be in church on sunday"?
I'm wondering that too! It does sound more natural.
Dé Domhnaigh is sunday in irish meaning God's day. The reference to the Sabbath day is perhaps a post reformation term implying Gods Day.
Gods day is derived from the latin deus die not a celtic words