Need to hide an element depending on date and post category

Drop this into the file that is displaying the date (assuming you are using get_the_date):

$month = date( 'F' );
$categoryIds = array();
$bannedMonth="September";
$bannedCategory = 13;
$categoryArray = get_the_category();
foreach ( $categoryArray as $categoryObj ) {
    array_push( $categoryIds, $categoryObj->term_id );
}
if ( $month != $bannedMonth && !in_array( $bannedCategory, $categoryIds ) ) {
    echo get_the_date();
}