pcbondsman (#6190) gave you the essentially correct answer but less than a better url. (The General is trying to be "diplomatic" here 'cause he hasn't forgotten PC's "acquaintance" with some wiseguys.)
If you go to Yahoo-Finance!'s Home page and scroll down, immediately below the bottom news headline you'll see Questions or Comments.
If you click that on and scroll down, ...and down, .....and down, you'll eventually get to something that reads as follows:
Average Daily Volume is the monthly average of the cumulative trading volume during the last three months divided by 22 days. It is updated weekly, and is provided by Market Guide.
Here's the link to that Q & C page of Frequently Asked Questions (FAQ):
Now by the General's reckoning, if you assume that every month has 22 weekdays (no less trading days, as, apparently, M-G does), that would mean that a year would consist of 12 x 22 = 264 days + 52 weeks x 2 weekend days/week = 104 days = 368 calendar days. It kinda leaves something to be desired, as the General sees it, to a more accurate ADV number.
So the obsessive/compulsive anal-retentive General calc's his ADV on the basis of the actual number of trading days within the past 3 months' worth of cumulative trading volume. To each, his own, as they say.
Fortunately for us, M-G isn't calc'ing any of NASA's space missions.
I knew you would have much more detailed info than I was able to provide on short notice.
Further, I also discovered in my 20 years with the military, (never attaining anything near the rank of General) that it's a good policy to let the "Brass" "one up you" whenever possible. Particularly so on those rare occasions when they do, in fact, known more than you do. :-)
But considering that they have PZN in the Business Services sector there's no telling how they do it. I suspect the General will have further comments on this and perhaps Yahoo statistics in, er, uh, general. :-)