Ran into this issue while trying to find a way to find the 30th of all months except Feb which needed to show the last day of the month. Created this formula below:

=IF(C2="FEB",EOMONTH(H5,0),CONCATENATE(MONTH(H5),"/","30","/",YEAR(H5)))

Where C2 shows the 3-letter abbreviation for the month in and H5 shows the first day of the current for the month listed in C2. The result is no matter what month and year is used in H5, if "FEB" shows in C2 it will always show me the last day of Feb or the 30th of all other months.