The following is an onOpen event handler for a form that makes the "Game center" button visible on the weekends:

function Form_onOpen()

   if new Date().getDay() % 6 == 0 // If today is a weekend day 

      this.gameCenterButton.visible = true // Enable access to game center page 

   endif 

The day number modulo 6 is zero for both day numbers 0 and 6, the days on the weekend.