“Easter Monday” and “Pentecost Monday” are not Christian religious dates. They’re like “Washington’s Birthday Observed,” which means, “Government employees would call in sick on the Monday if it were a scheduled workday.”
They are, respectively, the first days of the octave of those Feasts and were celebrated in most Catholic places, at least during the Industrial Rev. I was surprised myself to see that Pentecost Monday was also celebrated in Ireland until recent memory.