Re-Emergence Day

I'm trying to figure out the best way for us to come up with an "origin".

Some holidays have an initial first celebration that you can point to, some are commemorating a specific event.
However some, like the solstice, are labelling a specific day when a non-human phenomenon happens.
Long weekend-style holidays change their official timing year by year for convenience.

What about something that's got an intrinsic "birthday" charge to it. Like when you learn that somebody's birthday is on December 25th that's met with a chorus of sympathetic groans.

Like, your birthday just happens to fall on [TBD] Day. Y'know, the ONE day of the year where there isn't cake OR candles, because we're celebrating the day when [peoples] won their bloody independence by throwing all the cake in the [place]ton harbor?

I think it is fair to say we all know what re-emerged on this day. Not everyone celebrates it the same way, or even at all, but the original re-emergence itself was felt by everyone around the world. For some it had a very personal impact, for others it was of little import, but nonetheless, we all know.
My own life was changed in ways that might not be obvious to others, but the event still resonates with me even now, and that's why I celebrate it every year.