If the shepherds were tending their flocks that night, but you can bet it wasn't in December. Even in Palestine winters are cold, and the flocks and shepherd would be down in the village for protection against predators and exposure. Early Christians asserted May 20 as his birth date, and every month in the year has been advanced since.
As the Church began its campaign to "convert" the pagans in Europe, they found their authority undermined. In an age where life was generally nasty, brutish and short, "eat, drink and be merry" was celebrated at certain times of the year for thousands of years. One of the biggest the ancient pagan festival of Saturnalia, celebrating the passing of the shortest day by decking houses with holly and mistletoe