Rocket launches are notorious for not making deadlines. Especially the first launch of a new type (ISS launches, interestingly enough, have a very narrow window, usually a minute or three, so they eventually get to the point where they can launch on time).
Therefore any date given today, for a date in the future is utterly meaningless.
SpaceX will undoubtedly make a BIG deal about a Falcon Heavy initial launch. Of course, you will be able to check the SpaceX home page.
Generally, the most reliable source for launch info that I have found is at SpaceFlight Now's launch tracking page. Which as of this writing, does not even have a date listed for a Falcon Heavy launch.
Update: The SpaceFlight Now page, as of January 4th 2017, lists a "2nd Quarter 2017" launch for the Falcon Heavy Demo.
Update2: As of Jan 2018, the launch is expected any day now, as a Wet Dress Rehersal has completed, but a Static Fire is still pending.
Additionally first launch will be from Florida at pad LC-39A no longer at Vandenberg.