I'm sure for me it was the day about six years ago when my employer at the time had an IPO. I didn't make an nine-figure amount like the founders, but it was a very nice payday even as employee number 300-ish.
Of course, as the market giveth, the market taketh away. The biggest one-day drop I had was probably when that same company announced some earnings below market expectations a couple of years later, and the stock lost half its value overnight. I had been gradually selling it off prior to that in order to spread the tax bill out over a few years, so it wasn't the whole amount, and even after the drop it was still quite a bit above my strike price, but it still stung a bit.