So, you have no idea what will happen on March 19?
Didn't you get some idea from your sources you watch/read?
If you have no idea, anything that happens on March 19 could be used to say that is it; it happened. Perhaps that is what Q is doing - just throwing a date out there, knowing there is nothing, and saying whatever happens that day, thunderstorm, car wreck, another bomb set at a London airport, something else happens in the world, and he grabs that, saying that is IT; IT HAPPENED!