Edit in JSFiddle

var spans = $("#datepicker div span"),
    total = spans.length,
    fecha = $("input[name=fecha]"),
    day, month, year;

for (i = 0; i < total; i++) {
    if (spans[i].className == "day active")
        day = spans[i].innerHTML + "/";
    if (spans[i].className == "month active")
        month = spans[i].innerHTML + "/";
    if (spans[i].className == "year active")
        year = spans[i].innerHTML;
}

fecha.val(day + month + year);