The Christian Year
The early Christians who were mainly Jews were used not only to keeping one day in the week as separate but also to marking the year with certain religious festivals, notably Passover, Tabernacles, and Pentecost. From early times Christians kept a commemoration of Christ's resurrection. This was held at Passover time and was finally fixed on the Sunday following Passover. Pentecost was then celebrated at the appropriate time; the fifty days between the two were days of joy and rejoicing. The choice of 25 December (in the East, 6 January) for the birth of Christ is almost certainly because that day was the great pagan day of honor to the sun, and in Rome in the fourth century it was transformed into a Christian festival.