<form action="#">
  <input type="text" id="Day" name="Day">
  <input type="text" id="Month" name="Month">
  <input type="button" onclick="generatelink(); return false" id="generateLink" name="generateLink" value="Generate Link" class="button">
</form>

<div id="generatedlink">
    <u>Link</u>
    <a id="link" href="">Open App</a>
</div>

<script language="javascript" type="text/javascript">
function generatelink() {
    var day = document.getElementById("Day").value;
    var month = document.getElementById("Month").value;
    //alert(BaseURL);
    //http://yourserver.com/1_12.html
    var link = "http://yourserver.com/" + Day + "_" + Month + ".html";
    var a = document.getElementById("link");
    a.href = link;
    a.textContent = link;           
}
</script>