Coming back from Europe right before a major holiday such as Easter ought to increase the chances.

On an unpredictable basis, any time. Saw a very overbooked flight out of Frankfurt last fall but couldn't get bumped. Reason: We were upgrades in business class. Offered to be on any bump list but were told business class would not be called unless they were really, really needing volunteers. So sitting in business class turned out to be a mixed blessing. (Though in balance, certainly worth it, of course, and we were appreciative still of the upgrade -- but we just didn't get all the cookies in the cookie jar.)