This is the last part of an Ode of the Canon. The first part is the Irmos, then each troparia, then the final Katabasia.
This double Canon of the Nativity has both sets of Katabasiae. Sing the first set from Nov. 21 - Dec. 24. On Christmas, sing them both, as written here. Sing the second set from Dec. 26 - 31.
These Katabasiae are sung in Orthros (Matins) from August 1-13 and August 24-September 21. They can also be sung at the Sacrament of Baptism, while the baby or child is being clothed. They are sung in a special Tone 8, similar to the Canon of the Paraklesis.