This is the last day of the millennium with the same numeric day, month, and year. It'll be another 1000 years before you see 12.12.12 again.
Time will fly. You and I will not be here to see that (at least not in the same lifetime) ;). So enjoy this special day and make a promise to do something great for yourself (i.e. be kinder, do something that scares you, try something you never tried before, forgive someone, give your time to someone).
Whatever it is, do it with the intention of leaving your mark as having made a difference. And by that, I mean when you leave this world, at least a hand full of people were inspired by your existence. Happy 12.12.12. :)