This is very curious, since when do otaku have a festive day? Although it is necessary to clarify that it is not a day officially recognized by UNESCO or any other similar institution, it is simply the day in which this practice is claimed that every day gains more followers. In addition, despite being something of Japanese origin, both celebration and custom have spread throughout the rest of the world, especially here in North America and Latin America.
Now, what is an Otaku? Well, it is a term (of course Japanese) used in a popular and somewhat derogatory way to define a person of obsessive hobbies, which can be applied to any subject but has been used since its origin in 1983 mostly for obsessive fans of anime and the manga. Over time the term has been used outside of Japan simply as a definition to an amateur of Japanese popular culture, whether it is manga, anime or cosplay. (In my case it is mostly anime, manga and video games).
Excellent but how did the term originate? It was created by a writer named Akio Nakamori in Manga Burikko magazine for what was already mentioned in the previous paragraph. Since the 90s, this subculture began to grow in an unstoppable way until it became what is known today.
For several years, the subculture has its own day of celebration being December 15 (a celebration created by the otaku themselves), the term does not understand ages so it can be applied to both men and women without distinction.
As an additional curious fact, there is what could be called an otaku paradise, it is a city called Akihabara, which is an important commercial area of Tokyo, Japan. In this place entertainment related to anime and manga abounds, and it is one of the few places in Tokyo where it is common to find managers who speak English, Portuguese, Chinese, Hindi and even Spanish.
I remember that as a child I was already fascinated by anime and manga, however I did not know the term until one day my uncle observing that fanaticism told me:
Do you know what people who have that kind of taste are called? They are called Otakus.
I have nothing left to say: Happy Otaku day!