[Aetae]

<link rel="stylesheet" href="style.css">
<script>
(function(){
  var date = new Date(),
      time = +date,
      day = ( 12 - date.getDay() ) % 7 ;
  function friday(){
    document.getElementsByTagName('link')[0].setAttribute('href', 'friday.css')
  }
  if(day) {//не пятница
    date.setDate( date.getDate() + day )
    date.setHours( 0, 0, 0, 0 )
    setTimeout( friday , date - time )
  }else{  //пятница
    friday()
  }
})()
</script>

=))