How to limit the execution of a rule for sending an email to only run once in a day?
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.