How to retrieve the date on which a post was moved to trash?

When post is trashed, WP creates a meta field named _wp_trash_meta_time and it saves trashed time in that field. You can use that field to get trash time.

Edit:

Example:

$args = array(
    'post_status' => 'trash',
    'post_type'   => 'post',
    'nopaging'    => TRUE,
    'meta_query' => array(
        array(
            'key'     => '_wp_trash_meta_time',
            'value'   => array( strtotime("last week"), date('U') ),
            'compare' => 'BETWEEN',
        ),
    ),
);

Using this, you can get trashed posts which are trashed in last week.