Okay... So when did/does the CW launch?
As it applies to station affiliations: 9/18 is when all the stations rebranded and when the new affiliations took effect. 9/18 is when former UPN and WB stations that did not become CW stations began their new formats. (9/18 at 5am local time was when WB and UPN ceased to exist)
As it applies to the network itself, on 9/18 all the programming from the network which was formerly WB and UPN was branded CW
As for original programming, the CW network had their first original Network TV show on at 9pm eastern. Yes that programming was self-promotion, but it was an original program aired at the same time on multiple stations (definition of a network) and at the same time branded CW.
As to new regular series programming, the CW begins original series programming on 9/20.
So, if your definition of "launch" is when new series programs begin, then yes 9/20 is the launch date.
If your definition of "launch" is when the network begins to send programming with their new network brand at a regularly scheduled time and airs on all the affiliate at the time the network chooses, and when all the local affiliate begin to call themselves by the new network name, 9/18 was the day.
See ya
Tony