To enter, tweet a photo of your favourite college memory with the hashtag #JBJreunioncontest and tag your school.
Watch Jon's announcement on MTVU. (Since it's an American contest, you can only view this video if you're in the US, or if you use a proxy. I originally embedded the video in this post but it autoplays and I find that annoying.)
Jon also spoke to GMA about the Reunion Contest ahead of tonight's concert in Detroit:
UPDATE 30/03/17: This new graduation-themed video set to "Reunion" was just uploaded to YouTube. In addition to footage of people celebrating their graduations, it features snippets of commencement speeches from late Star Trek star Leonard Nimoy, Indiana University graduate Parker Mantell, and former US President Barack Obama: