Header image automatically changes depending on season

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');

}