track the process at the Journey North website.
During the migratory process, Monarchs often gather in tree "roosts" for overnight stays. Along the migratory route these are not nearly as spectacular as the enormous ones in Southern California or the ultimate ones at the Mexican terminus, but they are awesome spectacles nonetheless. I've had the good fortune to see one once while hiking. The map embedded above, from the Journey North website, shows the location of roosts reported so far.
Here is a brief amateur video of a tree roost. The video is low resolution, but it gives a flavor of the event, and the videographer has had the good taste not to spoil the images with some irrelevant music or commentary.