(coexisting on another dimensional plane)
A holy day was a day set aside for a religious observation. A holy day with special importance was called a high holy day.
In the mirror universe, the Terran Empire celebrated April 5, the day when Terrans made first contact with, and slaughtered the crew of, a Vulcan survey ship in 2063, as a holy day and not as a high holy day. (DIS: "Die Trying")