E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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 mans 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.