It is unknown in which month Jesus was born. Although his birthday is celebrated annually on December 25, the Bible offers some clues that the month of his birth may have been September
December 25 is celebrated as Jesus' birthday primarily because the date was already used as a pagan religious festival. Over the past 2,000 years there have been many suggestions as to the time of year in which Jesus was born but It was definitely not during December due to unfavorable weather. Luke 2:7-8 mentions that shepherds were in the fields tending to their flocks at that time, but sheep would not likely have been left out in the fields during the cold winter. Luke 2:1-4 explains that Joseph and Mary, Jesus' parents, traveled to Bethlehem for the Roman census. It was very unlikely that a census was held when winter would have made roads difficult to travel.
It is possible to guess at the birth month of Jesus based on Biblical accounts of the birth of John the Baptist. Since there were six months between the births of the two men, and John was born probably in March, the month of September in the best guess for when Jesus may have been born.