do_action( 'activate_header' ) Fires before the Site Activation page is loaded. ContentsSource Changelog User Contributed Notes Source File: wp-activate.php View on Trac Top ↑Changelog Changelog Version Description 3.0.0 Introduced. Top ↑User Contributed Notes Skip to note 1 content You must log in to vote on the helpfulness of this noteVote results for this note: 0You must log in to vote on the helpfulness of this note Contributed by Rnaby — 5 years ago activate_header hook is defined in wp-activate.php file which includes theme’s header.php but not the functions.php file. So it will throw fatal errors if any function or functions called on fire of this hook which are defined in theme’s functions.php. Also it’ll not work if you hook any function in functions.php. And this wp-activate.php page is only being called when WordPress multi site network is activate. You must log in before being able to contribute a note or feedback.