Puer natus est is the introit chant for the Third Mass of Christmas Day (Mass of the Day). The chant is in the mixolydian mode.

Find a list of other chant themes here.

Videos:
Olivier Messiaen – Improvisation “Puer Natus Est” – La Trinité
Tobias Schmid – Improvisation ‘Puer natus est’ – Rottenburger Dom
Jonathan Wessler – Fantasy-improvisation on Puer natus est nobis – Christ Church Episcopal, Rochester, New York