Modifies cron schedule in Wordpress allowing you to add custom schedules. From http://wp.smashingmagazine.com/2013/10/16/schedule-events-using-wordpress-cron/

modify_cron_schedules.php

add_filter( 'cron_schedules', 'wi_add_weekly_schedule' ); 
function wi_add_weekly_schedule( $schedules ) {
	$schedules['weekly'] = array(
		'interval' => 7 * 24 * 60 * 60, //7 days * 24 hours * 60 minutes * 60 seconds
		'display' => __( 'Once Weekly', 'my-plugin-domain' )
	);
	/*
	You could add another schedule by creating an additional array element
	$schedules['biweekly'] = array(
		'interval' => 7 * 24 * 60 * 60 * 2
		'display' => __( 'Every Other Week', 'my-plugin-domain' )
	);
  */

	return $schedules;
}