Question

wp_schedule_event($timestamp, $recurrence, $hook, $args);

$recurrence
(string) (required) How often the event should reoccur. Valid values are below. You can create custom intervals using the cron_schedules filter in wp_get_schedules().

    hourly
    twicedaily
    daily

    Default: None 

But what if i want to run my task once a week?

Answer 1

from the manual: http://codex.wordpress.org/Function_Reference/wp_get_schedules

add_filter( 'cron_schedules', 'cron_add_weekly' );

 function cron_add_weekly( $schedules ) {
    // Adds once weekly to the existing schedules.
    $schedules['weekly'] = array(
        'interval' => 604800,
        'display' => __( 'Once Weekly' )
    );
    return $schedules;
 }