The Japan Anniversary Association websites explains how today became to be "Twin Tail Day."
The explanation mentions that the hairstyle is admired worldwide and how titles like Sailor Moon and fashion brands have been spreading the appeal of the twin tail. The reason why its Feb the 2nd is because of how 2/2 (Feb 2nd) overlap with each other donating the Twin Tail.
...Sounds like a good reason to celebrate the bodaciousness of twin tails!
I always keep up with what goes on in Japan through Twitter Trends and discovered that I should have been celebrating all day today ><
But folks in the US still have a day ahead of them to break out the champaign.
Folks in Japan have been celebrating by drawing a load of twin tail girlies - check out this search query.