Interesting question because, for me, it all happened on an exact day. The day I knew I could retire.
I had very little investing knowledge and made lots of mistakes with investing BUT I had always been a good saver. So my retirement "stash" had grown - even though I hadn't particularly been good at investing. In fact, my Megacorp gave its 401(k) matching in company stock and toward the end of my c@reer, the stock had taken off and set me up with a nice nest egg. That, in itself was not enough to make me feel good about retirement. I would have kept w*rking had that been my only resource.
BUT on an exact date, my modest pension (non-COLA'd) and only about 45% of my salary vested AS DID my company provided Retirement Medical Insurance. I was only 51, so it was a long time to SS - even Early SS. It was even longer to MC BUT I felt very safe on the financial and medical aspect of retirement.
As far as how I felt: I literally relaxed and knew that I could march into the boss's office anytime I wanted to and say "I quit" and know that I would be just fine, financially.
Ironically. I was enjoying my j*b and decided to stay on for a while and enjoy the fruits of my l@bors. I had created a position for myself and I wanted to stay and see what I could do with it. I stayed for 7 more years, padding my savings and enjoyed myself until my assignment was changed. I retired the next Friday.
The relaxed feeling that came over me upon vesting was "palpable" (if that is an appropriate word for this mental transition). No one, and no Corp. BS could get to me anymore.
I felt "bullet proof." (What are they gonna do? Fire me? - No! I'd quit first)!! It was a very good feeling, indeed. YMMV