Today being Dominica in albis...

That is, the Octava Paschae, the octave day of Easter, I suppose that the fact that it is known by more than one name isn't surprising, given the great and solemn esteem the Paschal Feast has been held in by the Christian world for two millennia. It is the Octava, it is Quasimodo Sunday (because the introit begins, Quasi modo...), it is Low Sunday (for what reason I'm not sure, perhaps simply because by comparison to the glories of the long week of Easter Sunday, the succeeding Sunday is bound to be 'low', lesser in dignity, importance, ceremonial brilliance etc). I believe that in earliest times and first it was called the Sunday in albis depositis vel deponendis i.e. the Sunday when those baptized at Easter will have put off their white garments (on Saturday) and resumed in their ordinary clothing a more or less ordinary place in the pews (not that there then pews but...). And in these latter days, it is called also Divine Mercy Sunday, due to the influence of the cult of the divine Mercy and of Pope St John Paul. 

 Anyway, that was all preface to what I wanted to notice here: today the hymns are returned to the Office, the Opus DeiRex sempiterne caelitum (well, Hic est dies verus Dei in the 'reformed' office of Matins; so far as I can tell it is simply a different hymn, by St Ambrose and featuring in the Ambrosian Rite, than Rex sempiterne caelitum which the 'reformers' must not have much approved), Aurora caelum purpurat (Aurora lucis rutilat after 'reform': in this case, 'reform' was a return to the authentic text after Urban VIII's 'classicizing reform'), and Iam lucis orto sidere, thus far today. Let us see if YouTube will favor us with versions of each of these.

And Jam lucis orto sidere-- which, I had forgotten, is also attributed to Saint Ambrose:

Having consulted Fr Britt, Rex aeterne Domine is apparently the proper name of Rex sempiterne caelitum and is also one of St Ambrose's. Let's see if that isn't at YouTube. 

It is, under the name Rex sempiterne Domine. And now it's time for Terce and then breakfast; am not well up on Nunc sante nobis Spiritus but am pretty sure it has also been attributed to St Ambrose but may not actually be his. The text has survived through all the post-Tridentine 'reforming', too, I believe..