Yes, they announced it in several places.
However, for someone who doesn't play every single day, many of those can be missed. I knew that the start of the season was coming soon, but the specific date was not listed anywhere in the client. With the dynamic ads in the client, I'm sure it was posted at some point and I just missed it somehow.
I eventually went to the lol website and searched through the news articles until I found confirmation that it had started.
My suggestion is that Riot use its wonderful notification system to post a simple "Hey! Ranked placement matches have begun!" notification when you log in. It wouldn't take any extra coding, just 10 minutes to decide on what to say and post it.
I'm sure that the "Welcome to the 2016 season" video was intended to be posted on day 1 of ranked placement matches, but it didn't actually post until day 2 or 3... Leaving those of us with other commitments such as grad school in the dark as to the kickoff. Redundancy makes for happy players, especially when there is the chance that video production may take longer than expected ^_^