Stores category posts in an array

As Howard E said, without knowing more about what and how you want to reorder it’s difficult to give you useful advice. Here’s a good breakdown of the options and, here are some places to start.

1. If you want an array of posts to order your own way, you can use the native function get_posts() in order to create an array of posts based on selected arguments. More info here.
2. If you want to modify the existing loop on your archive page and order youor posts with custom arguments, you can use pre_get_posts()
3. If you want to write use existing WordPress arguments to order your posts, you can write a custom query using WP_Query and the ordering arguments it already supports.

Update

Since you want to sort your posts by a custom ACF field, here’s how to do that with get_posts():

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

ACF has an entire page dedicated to sorting by custom field. You shouldn’t need to put the posts back in an array in order to sort them.

Good luck!