template_redirect action-hook . WP 1.5.0

Fires before determining which template to load.

add_action( 'template_redirect', 'action_function_name_3877' );
function action_function_name_3877(){
	// action...
}

In file: /wp-includes/template-loader.php

template_redirect

do_action( 'template_redirect' );

add_action( 'template_redirect', array( $this, 'handle_render_partials_request' ) );

add_action( 'template_redirect', 'rest_output_link_header', 11, 0 );

add_action( 'template_redirect', 'wp_shortlink_header', 11, 0 );

add_action( 'template_redirect', 'wp_old_slug_redirect' );

add_action( 'template_redirect', 'redirect_canonical' );

add_action( 'template_redirect', 'wp_redirect_admin_locations', 1000 );

add_action( 'template_redirect', '_wp_admin_bar_init', 0 );

add_action( 'template_redirect', 'maybe_redirect_404' );