At the moment you have a few things wrong. The form action should be SaveEvent.php . Im not sure what the hiddenframe is but get rid of it. You also don't need to have an onsubmit function because you already have the action in the form.
<form name="event" action="SaveEvent.php" method="POST">
...
<input type="submit" value="Save" />
</form>
Then all the variables have been posted to SaveEvent.php and you would simply do the following on the SaveEvent.php page to initialize them.
$id=$_POST['EventID'];