function shiftSeason(){
	var north = true;
	var headerClass = ['summer','autumn','winter','spring'];
	var today = new Date();
	var month = today.getMonth();
	if (today.getDate() > 20) month++;
	var Season = headerClass[Math.floor((month+(north?6:0))/3)%4];
	document.getElementById('header').className = Season;
}

window.onload = shiftSeason;