The Octave of the Nativity of the BVM is a Simple Octave.
Simple Octaves are the Octaves of Feasts of the Second Class. These Octaves are neither celebrated nor commemorated within the Octave; only on the Octave Day itself the Office is of Simple Rite, and excludes the Saturday Office of the Blessed Virgin Mary, but admits the commemoration of an occurring Feast of Simple Rite.
~ Matters Liturgical (ed. 1942), 472 par. 8 (p. 302).