I'm a few days late on this, but Tau Day (6/28) was earlier in the week. Tau is a new idea to replace pi (the ratio of the circumference of a circle to its diameter) with the more natural constant tau (the ratio of the circumference of a circle to its radius). Seems crazy, but there's a pretty good case to be made for this.
The problem is that pi is so entrenched, I can't see it ever being changed. But it's interesting nonetheless. If I were still in the habit of writing mathematical papers I'd try to insert tau here and there (appropriately footnoted) and see if it caught on.