The phrase ‘day of days’ [Latin: die dierum], marks a particularly momentous day in the life of an individual or community. In the Christian tradition it is used to refer to the first Easter Day, when the reformist Jewish teacher, Jesus of Nazareth, who had been crucified, and lay dead and buried, was resurrected by the intervention of God.

The phrase in its Latin form, appears in the first line of the first hymn in Charles Coffin, 1676-1749, his Hymni Sacri (Paris, 1736):
‘Die dierum, principe / Lux č tenebris eruta: / Christus sepulcri carcere, / Lux vera mundi, prodiit.’
It was translated into English by Isaac Williams, 1802-1865; published in the British Magazine, April 1837, and in Hymns Ancient and Modern:
‘Morn of morns, and day of days! / Beauteous were thy newborn rays: / Brighter yet from death’s dark prison / Christ, the Light of lights, is risen.’
A decade earlier, John Keble, 1792-1866, published a hymn for Easter Day, in Christian Year, 1827:
‘O day of days! shall hearts set free / no ‘minstrel rapture’ find for thee? / Thou art the Sun of other days; / they shine by giving back the rays.’
This hymn was included in James Martineau, 1805-1900, his Hymns for the Christian church and home (London, 1840)

The phrase was also used by Christina Rosetti, 1830-1894, in her poem ‘I wish I could remember that first day, first hour, first moment of your meeting me.’
‘If only I could recollect it, such / A day of days! I let it come and go / As traceless as a thaw of bygone snow.’
Charles Darwin marked in his diary for 11 Nov 1838, ‘the day of days,’ it being the day that Emma Wedgwood accepted his proposal of marriage. [Origins: selected letters of Charles Darwin (Cambridge, 2008), p.68].