var once_per_session=1

function get_cookie(Name) {
  var search = Name + "="
  var returnvalue = "";
  if (document.cookie.length > 0) {
    offset = document.cookie.indexOf(search)
    if (offset != -1) { // if cookie exists
      offset += search.length
      // set index of beginning of value
      end = document.cookie.indexOf(";", offset);
      // set index of end of cookie value
      if (end == -1)
         end = document.cookie.length;
      returnvalue=unescape(document.cookie.substring(offset, end))
      }
   }
  return returnvalue;
}

function loadornot(){
if (get_cookie('popunder')==''){
loadpopunder()
document.cookie="popunder=yes"
}
}

function loadpopunder(){
win2=window.open("http://www.florida-vacations.com/xjadsvr/annc.aspx", 'remote', 'status,dependent,scrollbars=0,width=420,height=420,left=250,top=105') 
win2.blur()
window.focus()
}

if (once_per_session==0)
loadpopunder()
else
loadornot()

<!-- 
function Certify(URL) {
  popupWin = window.open(URL, 'Participant', 'width=400,height=350,menubar=no,resizable=yes,scrollbars=yes')
  window.top.name = 'opener';
}
function ShowCal(URL) {
  popupWin = window.open(URL, 'Calendar', 'width=200,height=225,menubar=no,resizable=no,scrollbars=no')
  window.top.name = 'opener2';
}