Noun
(idiomatic) A particularly noteworthy day; the day on which a milestone or especially memorable event occurs.
(Christianity, sometimes capitalized) Sunday, especially Easter Sunday.
Day of Days (uncountable)
Alternative form of day of days
Summoning artists to participate In the august occasions of the state Seems something artists ought to celebrate. Today is for my cause a day of days. Robert Frost
O Day of days when we can read! The reader and the book, either without the other is naught. Ralph Waldo Emerson