Actually, it would be difficult.  Adding is easy because the .hol file
containing the holiday definitions is a single source (which can be edited
with Notepad).  Once the holidays are on the calendar, however, it would
take VBA code to search through all the items on the calendar looking for
holidays and then removing all of those individual items one at a time.  In
some ways it's like a sorting mail in the post office.  You feed the
collected mail into a machine and automation moves all the mail to be
delivered to bins corresponding to the delivery routes, but there's no
mechanism of "unsorting" it other than manually moving all that mail back to
the starting point.  The automation simply can't run in the reverse
direction without a lot of work.  It's not an easily reversable operation.