jwposter:
We know that the 8th course served during the day of Atonement when Zacharias was serving in the Temple in 4 B.C.E.
It is only a tradition that this was on the day of atonement. It is not indicated in Luke (and the spurious story is only found in Luke), which was written after the temple was destroyed.
We simply plot the courses through the years backwards from 70 AD and see that the 8th course would be serving during the 2nd week of the 7th month in 4 B.C.E
Okay, I'll play... Day of atonement in 4 BCE was 2 October. There were 24 courses. Assuming 8th priest course on that date, by 30 August 70 CE (3804 weeks later) that gives us course... 21. Oh dear.
What about 5 BCE... Day of atonement in 5 BCE was 13 October. Assuming 8th course on that date, on 30 August (3855 weeks later) it brings us to... course 23.
So we can only conclude that the tradition about the day of atonement is just wrong.