I was going to list all of the free events, activities, performances, and prizes taking place this Saturday 9/12 to celebrate the opening of the region's newest MAX line. But there is just no way I could've listed everything.
So I suggest going over to TriMet's webpage showing the schedule of events and have yourself a field day! TriMet is hosting parties all over the city - Clackamas Town Center, Pioneer Courthouse Square, SE Fuller Rd MAX Station. Free Green Line MAX rides will get you to all the celebrations, and once you arrive there will be tons of stuff to do.
See you out celebrating!