Day Of Days Meaning
(Christianity, sometimes capitalized) Sunday, especially Easter Sunday.
Example: 1850, Edgar Allan Poe, "Morella":
"It is a day of days," she said, as I approached; "a day of all days either to live or die. . . . I am dying, yet shall I live."
1907, Jack London, "The Pen: Long Days in a County Penitentiary":
At last came the day of days, my release.
1940 March 11, "Sport: Four Hundred Grand," Time (retrieved 9 July 2015):
It was a day of days for California railbirds. Not only was it the day of the $100,000 Santa Anita Handicap, world's richest horse race, but this was the now-or-never race for doughty old Seabiscuit, darling of U. S. racing fans, Cinderella of the turf.
2012 Sep. 8, George Vecsey, "Tennis: Rain and Rest Days Spell the End of Super Saturday," New York Times (retrieved 9 July 2015):
Super Saturday has been the centerpiece of the United States Open for nearly three decades. But this day of days is heading toward a multimillion-dollar extreme makeover.
1863 June 7, "To the Editor: Sunday Railway Excursions," New York Times (retrieved 9 July 2015):
The residents skirting the line of the Harlem Railroad as far as Croton Falls, were not a little annoyed, last Sunday, by numerous cheap excursion trains. . . . [W]e doubt whether the law gave being to a great corporation for the purpose of tempting men from their homes on the Day of days.
2009, Charles G. Fuller, Give Him Time, ISBN 9781615070817, p. 107 (Google preview):
To be sure, Christians celebrate the Resurrection of Christ every Sunday, but there is a special place for the celebration we call Easter. It should be approached as the Day of Days.