Twelfth Night

Wikipedia? Love it. Today's main page notes that 6 January is Epiphany. What's that? Moreover, one thing leads to another... in this case to the bottom of the page. Wherein is the See Also section with its Twelfth Night. Twelfth Night? Well, that was the day we took Christmas decorations down when I was a kid. Twelfth Night? A play by William Shakespeare. But why did he name it so? Twelfth Night.