If you’re reading this between the times of 11:12 and 18:42 UTC (7:12 a.m. to 2:42 p.m. Eastern U.S. time) on Monday, then the transit of Mercury is occurring right now.
Get thee immediately to this page I wrote all about the transit to find out how to watch it live online, with your own equipment, or at a nearby astronomy club. This transit—when Mercury crosses the visible face of the Sun—happens pretty rarely; the next one won’t be until November 2019. So if you can, take this chance to take a look.
Remember: Never look at the Sun without proper protection! I have links to how to safely observe this on the other page. Go!