I go by March 28th.
I recognise the dates that others have proposed but there's just something about the day that this series, that my heart absolutely loves was born, was first unleashed unto the world that I really appreciate.
This year, when March 28th rolled by, I was devastated that nothing was released to celebrate it. No special artwork, no exclusive image, nothing.
Last year the anniversary was held on April 10th, and while that's a date special to me for personal reasons, I was worried Nomura would try and make it a date special for Kingdom Hearts. But even that didn't happen.
Regardless, I hold March 28th as the official Kingdom Hearts day. It's the day the series was properly born, it's Sora's birthday, and it's a day we all look forward to no matter what.