WordPress at a glance
action is not described

activate_header action-hook . WP 3.0.0

Fires before the Site Activation page is loaded.

Usage

add_action( 'activate_header', 'action_function_name_5959' );
function action_function_name_5959(){
	// action...
}

Where the hook is called

In file: /wp-activate.php
activate_header
wp-activate.php 78
do_action( 'activate_header' );

Where the hook is used (in WP core)

wp-includes/default-filters.php 521
add_action( 'activate_header', '_wp_admin_bar_init' );