// set the header for each season depending on todays month
var north = true; // northern hemisphere 
var backImg = ['summer.jpg','autumn.jpg','winter.jpg','spring.jpg']; //order is summer, autumn, winter, spring
var today = new Date();
var month = today.getMonth() + 1;

document.getElementById('header').style.backgroundImage = 'url(images/jpg/header_' + backImg[Math.floor((month+(north?6:0))/3)%4] + ')';