Use cookies to display modal on first visit. Code from http://acooke.web.wesleyan.edu/cookies.html and modified to load bootstrap modal.

bootstrapmodalonload

<script type="text/javascript">
//<![CDATA[

function createCookie(name,value,days) {
  if (days) {
    var date = new Date();
    date.setTime(date.getTime()+(days*24*60*60*1000));
    var expires = "; expires="+date.toGMTString();
  }
  else var expires = "";
  document.cookie = name+"="+value+expires+"; path=/";
}

function readCookie(name) {
  var nameEQ = name + "=";
  var ca = document.cookie.split(';');
  for(var i=0;i < ca.length;i++) {
    var c = ca[i];
    while (c.charAt(0)==' ') c = c.substring(1,c.length);
    if (c.indexOf(nameEQ) == 0) return c.substring(nameEQ.length,c.length);
  }
  return null;
}

function showModal() {
   jQuery('#loadModal').modal('show');
}

var visited = readCookie('mypopup');

if (!visited) {
  $(document).ready(function(){
    showModal();
    createCookie('mypopup','no',0);
  });
}

//]]>
</script>