Alex - Omniscope includes both a TODAY and an LASTDAYOFMONTH function, so it is easy to write a test formula of the form:

IF([TODAY]=[LASTDAYOFMONTH],true,false)

Omniscope features a Validate Data block which can be used to control flow based on evaluated results. As shown below and in the attached example, the above formula can be inserted in the Validate Data block such that it will return 'true' only on the last day of the month. The other days, it will return 'false' and when the Validate Data block is set to return Error on 'false', no data set will pass onwards to the Output block for Publishing. You can configure an error email to be sent saying effectively "Nothing published because today is not the last day of the month"

Note that the Validate Data block only accepts single value formulae, so the test formula above needs to be recast using:

IF(Subset_Singleton([TODAY]) = SUBSET_SINGLETON([LASTDAYOFMONTH]),true,false)

which evaluates the same as a single values rather than as multi-row results.

Thanks tjbate. Subset_singleton didn't work for me since I was working with more data rather than just a mapping table like your example, but Subset_last did what I wanted it to (because of the way my data was laid out) so I got what I wanted out of it.