How to theme the Event content type node creation form in drupal 6?
It all depends on what changes you want to make, but the standard way of altering forms in D6 (and D7) is invoking hook_form_alter() in a custom module. Do a switch on the form_id, and enter your edits, something like this:

YOUR_MODULE_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  // print $form_id; exit;
  switch($form_id){
    case 'event_node_form':
      // change the form. The following will alter the text on the submit button:
      $form['submit']['#value'] = 'Go!';
      break;
  }
  return $form;
}


If you don't know what the form_id is of the form you want to edit, just uncomment the print $form_id line and visit the form in your browser. You can refer to the D6 form api guide to find what form elements go with which properties. Hope that helps!