Oh! We just learned this via Gothamist:
Tonight is the first of the annual four Manhattanhenge events. Starting just around 8 p.m., the sun will align precisely with the Manhattan street grid, "illuminating both the north and south sides of every cross street of the borough's grid."
So be there tonight around 8:16 for the half sun on the grid; and 8:18 tomorrow for the whole shebang of the sun.
You may also experience this on 23rd, 34th, 42nd and 57th if you really want to.
The above photo is from 2009, when Manhattanhenge was so much better!