var dSS = {
names: ["Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"],
number: new Date().getDay(),
link: document.createElement('link')
}
dSS.link.rel = "stylesheet";
dSS.link.type = "text/css";
dSS.link.href = dSS.names[ dSS.number ] + ".css";
document.getElementsByTagName('head')[0].appendChild( dSS.link );
Назовите свои файлы css соответственно: Saturday.css, Friday.css, Sunday.css и т. д.