Is there a way to run an action where it only runs on the first Sunday of the month for instance or maybe the 15th of every month?
This is definitely possible if you leverage custom Relevance such as the following in conjunction with the existing applicability relevance:
Q: current date = first sunday of current month_and_year
A: False
Q: current day_of_month as integer = 15
A: False
This way your action would only be relevance if the applicability relevance is true AND one of the above is true.
How does that affect the scheduler?
The schedule part of the action only matters if the computer is relevant for the contents of the action (The fixlet or baseline).
If you add @Aram’s suggestion:
current day_of_month as integer = 15
then the action will only run on computers on the 15th of the month.
That means, if you specified the action to only run on a saturday or a sunday and then put:
current day_of_month as integer = 15 in the relevance it would effectively “AND” them together and only run if it was a saturday or sunday AND ALSO the 15th of the month.
Does that make sense?
I think this is what @Aram and @strawgate are suggesting…
When you Take Action open the Applicability tab choose the radio option that states ...the following relevance clause evaluate to true and add in the new clauses.
Say the existing relevance is
exists file "/foo/bar"
Add the new clauses so that new relevance is
(exits file "/foo/bar") AND (current date = first sunday of current month_and_year AND current day_of_month as integer = 15)
Although the first sunday will never be the 15th day 
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