var header = document.getElementById("header");
if(header){
	var time = new Date();
	var fileName = 'header-snow.gif';
	if(time.getHours() > 21){
		fileName = "header-night.jpg";
	}else if(time.getHours() > 18){
		fileName = "header-evening.jpg";
	}else{
		fileName = "header-day.jpg";
	}

	header.style.background='url(http://www.katkovonline.com/wp-content/themes/igor-katkov/images/' + fileName + ') no-repeat';
}