During the spring (in March or April) there's a Jewish festival called 'Passover'. This festival remembers when the Jews had escaped from slavery in Egypt about 1500 years before Jesus was born. Lots of lambs would have been needed during the Passover Festival, to be sacrificed in the Temple in Jerusalem. Jews from all over the Roman Empire travelled to Jerusalem for the Passover Festival, so it would have been a good time for the Romans to take a census. Mary and Joseph went to Bethlehem for the census (Bethlehem is about six miles from Jerusalem).