//CSSのpathを書く。最後は/で終わる。
var CSSPath   = 'css/';

var ThisDate  = new Date();
var ThisMonth = ThisDate.getMonth() + 1;

switch(ThisMonth){
  case 3: case 4: case 5:
    var CSSFile = "spring.css";
    break;

  case 6: case 7: case 8:
    var CSSFile = "summer.css";
    break;

  case 9: case 10: case 11:
    var CSSFile = "autmn.css";
    break;

  case 12: case 1: case 2:
    var CSSFile = "winter.css";
    break;

  default:
    var CSSFile = "spring.css";
}

document.write(unescape("%3Clink rel='stylesheet' href='" + CSSPath + CSSFile + "' type='text/css' /%3E"));