passing user input variables in a post request with javascript

here is my form:

<form type="post" id="calendarform">
      <label for="day">Day</label>
      <input name="day" id="day" type="text"></input>
      <div class="clear"></div>
      <label for="time">time</label>
      <input name="time" id="time" type="text"></input>
      <div class="clear"></div>
      <label for="event_text">Event</label>
      <textarea name="event_text" id="event_text"></textarea>
  </form> 

I want to submit the user inputed variables to a php script via a post request using an xmlhttp object in javascript. Here is the code I'm using to try and do that, which isn't working:

function set_event(){
  var day = document.getElementById("day").value;
  var time = document.getElementById("time").value;
  var event_text = document.getElementById("event").value;


  var xmlHttp = new XMLHttpRequest();
  xmlHttp.open("POST", "set_event.php", true);
  xmlHttp.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
  xmlHttp.send("day=day&time=time&event_text=event_text");
  alert("setfired");
}

The POST variables are being instantiated but they're not being set. Any help would be much appreciated.