In the Talmud does it describe the Order of the Priest? In II Chronicles, 24 lists, the division of the sons of Aharon. In the "Encyclopedia of Jewish Concepts" by Philip Birnbaum, he states that the priest rotated serving one week at a time. Therefore, that would account for only 48 of the 51 weeks. And how did they account for leap years when there was an extra month? Was the rotation tied so that it begun on the 1st of Nisan?

The rotation was not tied, it just went on and on all year round, and when the rotation finished they started again regardless of the date. Rabbi Melamed, Machon Hamikdash