Fire event after client side script validation in asp.net?

The ASP.NET calls the WebForm_OnSubmit, thats runs the validation. After validation is ok, its continue and run the rest JavaScript functions that's found on onsubmit on the form.

So to execute a JavaScript after the side validation, just place it on the form tag.

For example:

<script>
  function CallMeAfterValidation()
  { 
    // here is the place to run your code
    return true;
  }
</script>

<form onsubmit="return CallMeAfterValidation();" runat="server" ...></form>