a simple example:
$holidays = array(
'01-01' => 'newyear',
'02-14' => 'valentines',
'03-17' => 'saintpatricks',
'04-12' => 'easter',
'12-25' => 'christmas',
);
$date = date('m-d');
if (isset($holidays[$date])) {$logo = 'logo_' . $holidays[$date] . '.gif';}
else {$logo = 'logo_default.gif';}
echo '<img src="' . $logo . '">';