FileMaker does not evaluate 3/2/2015 as a date. It evaluates that value as 3 divided by 2 divided by 2015. Note that / is the division operator.
You could use:
YourTable::Event_Date > Date ( 3 ; 2 ; 2015 ) And YourTable::eventDate < Date ( 3 ; 15 ; 2015 )
but I suggest using a reference to global date fields in place of the date functions so that you can filter for different date rangs by changing the value of the global field instead of having to redefine the portal filter.