Try
var months = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun', 'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec'], flag = {}
$('.container > .child').each(function () {
var month = this.className.match(/(\d{2})-(\d{4})/);
if(flag[month[0]]){
return;
}
flag[month[0]] = true;
$(this).before('<div class="header">' + months[month[1] - 1] + ' ' + month[2] + '</div>')
})
Demo: Fiddle