How to show the posts of some category first, and then all other

How to show the posts of some category first, and then all other posts that does not belong to that category?

I have logic that get the category and reorder the posts. But i don’t know how to use. Please help.

$args = array(
    'post_type' => 'event',
    'meta_key'  => '_event_end_date',
    'meta_compare' => '>',
    'meta_value' => $today,
    'order' => 'DESC',
    'posts_per_page' => '10'
);
$loop = new WP_Query( $args );

You can’t do it in one query. You have to add second query to show the posts of another category. Like

$args = array(
    'post_type' => 'event',
    'cat' => 4 //category id.
);
$posts = new WP_Query( $args ); // Get all the posts category which you want to show first.

$args2 = array(
    'post_type' => 'event',
    'cat' => -4 //Display all posts exclude this category.
);
$posts = new WP_Query( $args2 ); // Get all the posts category which you want to show second.

For reference check here: http://codex.wordpress.org/Class_Reference/WP_Query#Category_Parameters