Where in the Bible does it say Christ was born on 12/25?
The simple answer to this is..... it doesn't. This date originates from one of the early church writers, Hippolytus. It was also recorded in some of the early Roman documents as a celebratory day of Pagan rituals. Christians not wanting to have anything to do with these rituals made it a day of celebration of Jesus Christ.
The reality is we really don't know the exact date Christ was born. Some argue that it couldn't be in the winter because to was too cold to care for flocks that were in the field, but there are several instances of people tending to their flocks at this time of the year, so that argument can not be made. As stated above, we just don’t know the exact date.