The dates published online were speculative and not confirmed by Samsung.
The S21 Series is, I believe, just starting in the last day or two and will be rolled out in batches over a number of days / weeks and to unlocked versions first.
If your device came via a carrier / network, you'll need to wait until that network has approved the software for release which can be a few days or can be a few weeks depending the the carrier and testing.
I am not employed by Samsung nor do I receive any remuneration as a Samsung Members Star.
My device is an S25 Ultra.