Change filter call
add_filter('masthead','seasonal_site_header', 10, 2);
function seasonal_site_header ($classes="") {
$hemisphere="northern";
$m = date('m');
$northern=array(
'summer' => array(6, 7, 8),
'fall' => array(9, 10, 11),
'winter' => array(12, 1, 2),
'spring' => array(3, 4, 5)
);
foreach($$hemisphere as $key=>$val) {
if(in_array($m, $val)) {
$classes[] = $key;
return $classes;
}
}
return $classes;
}
Change css
#masthead.site-header.winter
{
background-image: url('/wp-content/uploads/2020/09/winter.jpg');
}
#masthead.site-header.spring
{
background-image: url('/wp-content/uploads/2020/09/spring.jpeg');
}
#masthead.site-header.summer
{
background-image: url('/wp-content/uploads/2020/09/summer.jpeg');
}
#masthead.site-header.fall
{
background-image: url('/wp-content/uploads/2020/09/autumn.jpg');
}