Show lightbox every X visit to site

Michelle, here's a handy utility function that lets you check if a visitor is on their ​n​th visit to the site (where visit is defined as a new browser session):

<script>
(function(global) {
  var propCurrent = 'MM_Session',
    propAll = 'MM_SessionHistory',
    currentSession = sessionStorage.getItem(propCurrent),
    allSessions = JSON.parse(localStorage.getItem(propAll) || '[]'),
    ts = new Date().getTime();
  if (!currentSession) {
    currentSession = sessionStorage.setItem(propCurrent, ts);
    allSessions.push(ts), localStorage.setItem(propAll, JSON.stringify(allSessions));
  }
  global.isEveryNthSession = function isEveryNthSession(n) {
    return (allSessions.length % n == 0);
  }
})(window);
</script>

You use the function like this:

if ( isEveryNthSession(4) ) {
  // do something on every 4th visit, like pop up a lightbox
}