This funky punctuation mark is, as it appears to be, a combination of the question mark and the exclamation point and is used in situations where you are expressing both a question and surprise/disbelief at the same time such as “You think the Canucks should trade Roberto Luongo for a 4th round draft pick‽” or “wtf‽” The alternative to the interrobang is to use both punctuation marks, (e.g., “wtf?!” but that’s considered poor style. And is used by people who aren’t cool enough to know about the interrobang.
¹Yes, I realize that I’m Canadian and the “National” in “National Grammar Day” appears to refer to the USA, but if the National Hockey League can call itself “National” despite being made up of teams in both Canada and the US, then I’m willing to honour NGD here in the Great Rainy North.²
²I’m in Vancouver, so it’s not really the Great “White” North.