There was also the initial announcement:
https://steemit.com/hardfork20/@steemitdev/hardfork-20-testnet-details
Related github:
https://github.com/steemit/steem/commit/3a0f9448bb1e59af7835e444c80ea094aab627c2
So that one activated HF20 on 2018-08-27 17:00:00 UTC. I remember there being two. I guess there were three. I probably got that mixed up because of the halt.
If even you got mixed up.... ;)