You know what's really missing from the whole holiday experience in the US? It's not more shopping time, that's been expanding for years. It's mid-July and I'm expecting to see Christmas decorations at the stores any time now.
No, it's an official drink for Halloween. That was going to be your next guess, I know. I don't know if you've ever taken your kids out begging for candy but it's a loud, chaotic affair. A perfect excuse to extend the limoncello-drinking season well beyond summer.
This cocktail is all class. The color is faintly reminiscent of blood, but not cheesily so. The glass is perfect for holding while observing others bob for apples. The sprig of rosemary marks you as a discerning, dignified adult who just happens to be wearing a Barney costume right now.
The drink is named the AB Negative because it establishes the blood/Halloween connection, but also because it's the rarest of all the blood types. Only 0.6% of the population has that blood type--it's true, I just Googled it. That amounts to 1 in 167 people, which somehow sounds far less impressive than 0.6%.
If you confine consumption of this cocktail to Halloween (that's impossible, I know, this is just a thought experiment), then you can only drink it 0.003% of the year. Now THAT'S an impressively rare and uncommonly delicious treat!