Modify wp_query for a specific category in category.php

Suppose you have a category called “events“, with a custom field with the date of the event, which is different from the post date. If you want to order the loop based on that field, only for that “events” category, you can do as follow.
Place it in category.php, at the beginning (or before the loop):

global $wp_query;
if($wp_query->query_vars['category_name']=='events') {
	$args = array_merge( $wp_query->query_vars, array(
		'orderby' => 'meta_value_num',
		'order'=>"DESC",
		'meta_query' => array(array(
			'key' => '_event_date',
			'value' => 0,
			'compare' => '>',
		))
	));
	query_posts($args);
}