Its a very good question, and I had not thought of it myself until it came up in this thread where I wondered the same thing.
https://jamfnation.jamfsoftware.com/discussion.html?id=14619

I honestly don't know if the JSS sees a month as when it goes from one calendar month to the next, or if it uses a set number of days.
I may end up setting up a test once per month policy to run on a test Mac on July 1 and then see when it runs again, unless someone comes up with the answer before then.

Why not just contact JAMF Support and ask them for clarification? I'm sure the documentation/manual could probably use it. Or you could test it too like @mm2270 suggested....

From my really bad memory I think it's a rolling 30 days.... but I am only about 75% sure

C

I have 8 computers in a monthly execution policy already. Been a few weeks since it was applied the first time, I'll keep an eye on the policy to see when it gets executed the second time.

I have a policy that runs weekly, I checked it and based on policy logs for one of the computers its executing every 7 days. Just a few more days to wait and see when the monthly policy runs

Yeah, well, a week is a week is a week. No surprise that's running every 7 days. Months are more variable so it will be interesting to see when that one fires off next. Keep us posted. The curiosity is rising. 'Course, we could just call up JAMF as @bpavlov mentioned, but... it seems more fun this way :)

so it looks like Frequency of Once every Month occurs every 30 days.

For a lab of 8 macs I have 3 policies that have frequency of once every month, the first execution was on 5/16 -- then on 6/14 the policy executed for a second time on 7 of the lab computers (1 was turned off), after turning on the single computer, the policy later ran.

Ah.. well, thanks for following up and letting us know @tcandela . That's actually a bit disappointing. Over time the "monthly" execution will start to drift and may even end up with 2 executions within a calendar month with a rolling 30 days. That will depend on when you start the policy. If first execution is set for the 15th of a month, it will take a while before they start to overlap, but if set for the 30th of a month, its not going to take long for things to get messed up.

It would be more useful to have a setup like a calendar repeating event:

Every 1st Monday of the month.
First day of the month.
Etc.
But I'm sure the smart guys can do that in other ways...