David Organizes the Priests and Levites Into Courses (1 Chronicles 24) December 17
Twenty-four chief men are chosen to head up the courses of priests who are to serve in some sort of rotating fashion at the temple. Note that Ahimelech, the son of Abiathar, has taken over his father's priestly duties, which has apparently been the case for some time (compare 18:16; 2 Samuel 8:17), even before Abiathar's siding with Adonijah.
Of some interest is the eighth course, given to Abijah (1 Chronicles 24:10), in whose division John the Baptist's father Zacharias was serving some 1,000 years later when he received his visit from the angel Gabriel regarding John's birth (see Luke 1:5-23). When their service was completed, the priests in each course could return home to their families, as Zacharias did (Luke 1:23).
Tradition tells us that each course served for one week at a time (except during the three festival seasons, when all courses served together). And Scripture gives an indication that this was likely the case. While dealing specifically with the gatekeepers (who were Levites, but not priests), 1 Chronicles 9:25 states that each division of them served for seven days. That this was probably also the case with the priests is indicated by the fact that the Levites assigned to serve at the temple were also divided by lot into 24 courses (24:19-31).
Lining these weeks up on the Hebrew calendar gives indication as to when Zacharias servedand thus when John the Baptist was conceived, as well as Christ, who was conceived six months later (compare Luke 1:26, 36). This would put John's birth in the spring of the year and Christ's birth in the autumn. (For more details, see "Why Jesus Christ Wasn't Born on Dec. 25," Holidays or Holy Days: Does It Matter Which Days We Keep?, p. 8).