Although most Christians celebrate December 25 as the birthday of Jesus Christ, few in the first two Christian centuries claimed any knowledge of the exact day or year in which he was born. The oldest existing record of a Christmas celebration is found in a Roman almanac that tells of Christ’s Nativity festival led by the church of Rome in 336 A.D. The precise reason why Christmas came to be celebrated on December 25 remains obscure, but most researchers believe that Christmas originated as a Christian substitute for pagan celebrations of the winter solstice.