day of days

noun2

1. •
   A particularly noteworthy day; the day on which a milestone or especially memorable event occurs.

   • "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." — 1850
2. •
   Sunday, especially Easter Sunday.

   • 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. — 2009