What is the error?

function day(){
date = document.multistring.txt1.value;
var sup = "";
if (date == 1 || date == 21 || date ==31)
{
sup = "st";
}
else if (date == 2 || date == 22)
{
sup = "nd";
}
else if (date == 3 || date == 23)
{
sup = "rd";
}
else
{
sup = "th";
}
var month = document.multistring.txt2.value;
var year = document.multistring.txt3.value;

alert("Today is " date "<SUP>" sup "</SUP> " of month year);

}

<Input Type = Button VALUE = "Click here" onClick = "day();">

Ya its working. Had some minor mistakes. Thnx dear

It is working in my code.
It means you do something wrong in diff place.
you may check it in diff one.

I have the same code both in my Head and body section, but its not working.

copy this code to your head section
<script>
function date() {
var date=document.frmone.txt1.value;
var month=document.frmone.txt2.value;
var year=document.frmone.txt3.value;
var sup ="";

if (date == '1'|| date == '21' || date =='31')
{
sup = 'st';
}
else if (date == '2' || date == '22')
{
sup = 'nd';
}
else if (date == '3' || date == '23')
{
sup = 'rd';
}
else
{
sup = 'th';
}
alert('Today is ' + date + sup +' ' + month + ' of '+ year );

}
</script>
and copy it to your body section U'll get the result.
<form Name=frmone>
Date:<input type=text NAME=txt1 SIZE=10 value=""/>
Month:<input type=text NAME=txt2 SIZE=10 value=""/>
Year:<input type=text NAME=txt3 SIZE=10 value=""/>
<input type=button NAME=b1 value="Complete Date" onClick="date();"/>
</form>

Hope this ll solve ur prb.