Many people will turn out in droves to witness a rare and spectacular astronomical event: a transit of Venus across the disc of the Sun. This will be only the 7th such event ever recorded, and the next one will not occur until 2117, 105 years from now.
Full story here.
Predicting and observing these events was once considered one of the highest accomplishments in science, not least because of the very rare nature of them. Scientists went to the ends of the Earth in search of ever more precise measurements.
There is some speculation the the ancient Mayan civilization knew about these transits, and was able to predict the one this year many centuries in advance. It depends on how you interpret some of their inscriptions. If this is true, it would mean that their knowledge of astronomy far exceeded the standards of the time, and rivals our knowledge today.
In any case, I encourage all of you to find a clear western horizon, get a filter or other viewing equipment, and enjoy the show. More information on when you can best view the event can be found here.