For a week or so around the Spring and Fall Equinox (around March 21 and September 21 most years), you can see the sunset looking down the east-west streets of downtown Chicago. The sunset is framed perfectly between the buildings on each side of the road. You should be able to see "Chicagohenge" for at least a few more nights this week.
Why does this happen?
Throughout the year, the sun sets at different points on the horizon. In the winter it is a little farther to the north and during summer it is a little farther to the south. But during the equinoxes, with the east-west grid layout of the city, we have the perfect opportunity to see "Chicagohenge".