If you say something ends on midnight the 19th, that means the 19th is still included. Similarly, the client communicated the 19th to be the last day of the ranked season. It is not if the ranked season ends as soon as the 19th starts. If Tuesday is the last day of your holiday, you return to work on Wednesday, not on Tuesday.
Also, the service announcements seem to be confused as well. One says the ranked season has ended, and another announces that ranked queues will be turned of the 20th during the early hours. But they're already turned off now.
If I knew this I wouldn't have played my placements the 18th. I'd rather be unranked than be placed in silver and no chance to win a few games and get out.