Source

File: wp-includes/class-wp-rewrite.php

public function get_month_permastruct() {
		$structure = $this->get_date_permastruct();

		if ( empty( $structure ) ) {
			return false;
		}

		$structure = str_replace( '%day%', '', $structure );
		$structure = preg_replace( '#/+#', '/', $structure );

		return $structure;
	}