add_filter( 'woocommerce_coupon_is_valid', 'coupon_week_days_check', 10, 2);

function coupon_week_days_check( $valid, $coupon ) {
	$coupon_code = "";
	// WooCommerce version compatibility
	if ( version_compare( WC_VERSION, '3.0', '<' ) ) {
		$coupon_code = strtolower($coupon->code); // Older than 3.0
	} else {
		$coupon_code = strtolower($coupon->get_code()); // 3.0+
	}
	// 'discountcode' => array of days when the discount code can be used
	$coupon_code_wd = array(
		'coupon20' => array('Tue', 'Thu'),
	);

    // if the coupon is one of those to check
    if ( isset($coupon_code_wd[$coupon_code]) ) {

    	// get the coupon days of validity
    	$valid_days = $coupon_code_wd[$coupon_code_wd[$coupon_code]];
		
		$now_day = date ('D');

		// if is not a valid day
		if( !in_array($now_day, $valid_days) ) {
			$valid = false;
		}
    }

    return $valid;
}