(The) means Christmas Day, the anniversary of the birth of Jesus.
The Cave of the Nativity is under the chancel of the “church of the Nativity.” In the recess, a few feet above the ground is a stone slab with a star cut in it, to mark the spot where the Saviour was born. Near it is a hollow scraped out of the rock, said to be the place where the infant Jesus was laid.
To cast a man's nativity is to construct a plan or map out of the position, etc., of the twelve houses which belong to him, and to explain the scheme.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894