Jens are the fictive currency used at Japan Expo: you win them at games and other activities, and then exchange them for manga, figurines, T-shirts, and many other items. Jens usually have no expiration date and are valid in all Japan Expo festivals.
There are some changes in 2017 with new Jen notes, different from the old ones. Those new Jens will work the same way but the old Jens will only be valid for one more year, 2017, and Japan Expo 18th Impact is your last opportunity to use them.
So it’s time to find the Jens you’ve collected so far and treat yourself by exchanging them at Japan Expo!