modify this in your js function :
function Validate()
{ var CheckInDate = document.getElementById(CheckIn).value;
var CheckOutDate = document.getElementbyId(CheckOut).value;
if var CheckInDate = "" || var CheckOutDate = "" {
alert ("One or more fields empty");
return false;}
return true;}
and in input tag in design :
<input type="submit" name="Submit" id="Submit" value="Submit" onclick="return Validate()" />
Hope this will help you.
Don't forget to mark as answer if it helps. :)