meta_by_date

$meta_query = array(
	'relation' => 'OR',
	array(
		'key'     => 'kk_events_event_start_date',
		'value'   => 'date',
		'compare' => '>=',
		'type'    => 'DATE',
	),
	array(
		'key'     => 'kk_events_event_end_date',
		'value'   => 'date',
		'compare' => '<=',
		'type'    => 'DATE',
	),
);