It's happening today, starts about half an hour after this post. Visible from Europe and America (even west coast America once the sun rises).
As always, don't view the sun directly - you need eye protection or a projector method. Mercury is pretty tiny, smaller than some sunspots, so if you just have ordinary solar glasses with no magnification you'll need to have pretty good eyes to see anything.
Best way to watch, for most, is to use one of the live on-line streams.
It's the last one before 2049. I probably won't be around for that one.