I get a little giddy talking about this — the date most folks treat as Hatsune Miku’s official birthday is August 31, 2007. That’s the day Crypton Future Media released her first Vocaloid voicebank for Vocaloid 2, and fans treat that release date like her birth anniversary. The voice behind her initial bank is Saki Fujita and KEI designed her iconic teal-twintail look, so August 31 is basically the date the character, voice, and look all became public.
That said, there’s a secondary, wildly popular celebration on March 9 — 3/9 reads as “mi-ku” in Japanese wordplay, so it’s affectionately called Miku Day. You’ll see different vibes around each date: August 31 feels like an anniversary party for the software/character, often tied to events like 'Magical Mirai', while March 9 is a cute meme-driven day where fans flood timelines with art, remixes, and puns. Personally, I love both — one’s official and historical, the other is pure fandom energy, and I join in for the art drops every year.