Programmatically send newsletter in hook_cron

This sets the sent status of the newsletter to pending. The newsletter will be sent during the next cron runs.

function yourmodulename_cron() {
  $nid = 1;
  $node = node_load($nid);
  if (yourcondition) {
    db_query("UPDATE {simplenews_newsletters}
                     SET s_status = '1'  
                     WHERE nid = %d", $node->nid);
    /* Uncomment the line below if using Simplenews 6.x-2.x */
    /* module_load_include('inc', 'simplenews', 'includes/simplenews.mail'); */
    simplenews_send_node($node);
  }
}