It's a twice-in-a-lifetime thing, today's passing, or transit, of Venus across the face of the Sun. The last time it happened was eight years ago, and after today it won't happen again for more than 100 years (and this pattern -- two transits about eight years apart, with each pair of transits separated by more than a century -- is exactly how it has always been, and will be).
So hold tight from 3:07 p.m. to almost 9:30 p.m. Pacific Time (and at the same corresponding time elsewhere) today. Assuming clear skies, proper geographic location (click here and use the options to see where you fit in), and the right eye-wear (or access to a computer webcast like the one found by clicking here), Venus "will appear as a small black dot gliding across the disk of the sun." And Venus's glide will have a royally exquisite pace: given its six hour plus total transit time, the planet from our earthly perspective will slowly inch across the face of the sun.