Учитывая этот HTML:
<input type="text" id="month">
вот js:
var months = ["January", "February", "March", "April", "May", "June",
"July", "August", "September", "October", "November",
"December"];
$("#month").blur(function() {
$(this).val(months[parseInt($(this).val())-1]).fadeIn();
});