Keep a list of holidays days by day-of-the year, rather than month/day (you could even do it by epoc time instead). Here is some code that might help:

use warnings;
use strict;
use POSIX;

my $yday = (localtime())[7];

print "Today is $yday\n";

my $mday  = 25;
my $month = 12;
my $year  = 2010;

my $time_t = POSIX::mktime(0, 0, 0, $mday-1, $month-1, $year-1900);

my $cday = (localtime($time_t))[7];

print "Christmas day 2010 is day $cday\n";
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You can still store these day numbers as a hash key (with an undef value).