What hook should I use that will fire whenever I open a post for editing in the WP back-end?

Don’t what you want to do but here is something that I did earlier on editor page of post type.

function wpb_change_title_text( $title ){
    $screen = get_current_screen();

    if  ( 'post' == $screen->post_type ) {                //Specify post type
        $title="New Placeholder text will come here ";  //Enter new placeholder text
    }

    return $title;
}

add_filter( 'enter_title_here', 'wpb_change_title_text' );

This function will change placeholder text of title textbox of any post type, here I am changing text of post (dashboard -> add post).