day is a WordPress query string variable that can used to request archive of posts published on a certain day, across months and years.
day global variable stores day of the current post. It is set in the function
setup_postdata($post) defined in [wordpress root folder]/wp-includes/functions.php.
WordPress uses this global variable internally, e.g., to form the HTML title when any archive is requested. Plugin and theme developers can use
day to get day of the current post.
Back to full list of global variables.