Every four years, we celebrate Leap Day as the shortest month of February gets extended by an extra day and we have to cope with the idea that 29 February is just like any other day.
That's an inaccurate statement, because there are years where you would think there might be a 29 February inserted into the calendar, but isn't. Those are years that can be divided evenly by 100 and the next time that 29 February will be left out of the calendar after a four year interval will be in 2100.
But that's not quite right either, because there are plans to insert a 29 February into the calendar for 2400. And every 400 years after that.
It's all very complicated, but fortunately in the leap year of 2016, Matt Parker took the time to not only explain why 29 February keeps getting added to the calendar when it does and why it doesn't when you think it should, but also how to fix it so calendars make more sense.
Glad to have that all sorted! Now if we can just get him working on fixing daylight savings time.