How do I explain the date of the Immaculate Conception of Dec 8 and then the birth of Jesus on the 25th
I think we have a misunderstanding here. The December 8th Solemnity of the Immaculate Conception refers to Mary's conception, not Jesus. Catholics believe that Mary was preserved from the stain of original sin from the moment of her conception.
The conception of Jesus is celebrated by Catholics on March 25th in the Solemnity of the Annunciation and that is exactly 9 months before Catholics celebrate the birth of Jesus (December 25th).
However, its important to note that Catholics celebrate those events on those days but do not hold for certain that those events actually happened on those dates.