var now = new Date();
var mon = now.getMonth()+1;
if( mon < 3 ){
	$("#header").css("background-image","url(/img/nav_bg.gif)")
}
else if( mon < 6 ){
	$("#header").css("background-image","url(/img/nav_bg_haru.gif)")
}
else if( mon < 9 ){
	$("#header").css("background-image","url(/img/nav_bg_natsu.gif)")
}
else if( mon < 12 ){
	$("#header").css("background-image","url(/img/nav_bg_aki.gif)")
}