How to limit the execution of a rule for sending an email to only run once in a day? I have a rule that sends an email out to subscribers on the 28th of each month. The rule is executed when cron is run, I have cron set to automatically run once a day.

I also use linkchecker, which uses cron to check the links on the site. Right now I have to remember on the 28th of the month not to run cron for the linkchecker, otherwise my subscribers will get multiple emails that day for each time cron was run. 

How can I limit the rule to only run once in any specific day, so that I don't have the potential to accidentally run cron manually and spam my subscribers?
Have a look at the "Rules Once per Day" module. Here is how it works (as per the comment in issue 2495775, from the module owner):


You specify a trigger hour on the administration settings page for this module.
The Rule trigger will then run when cron tasks are first run after the start of that hour. The actual run time will depend on your cron task timings.


So this is another way to understand/Read this:


The "Event" will only be triggered when a cron job is run.
And that event will only be triggered 1 time / day, i.e. "next time cron runs after the trigger hour has passed".


With that, you should be able to adapt your "rule that sends an email" to run ... once a day, on any day, including the 28th of each month.