// Refer to meta_key (name of the ACF field) and orderby meta_value

$posts = get_posts(array(
	'post_type'			=> 'event',
	'posts_per_page'	=> -1,
	'meta_key'			=> 'the_field_name',
	'orderby'			=> 'meta_value',
	'order'				=> 'DESC'
));