Download the Citi Bike app and you can select a free 24 hour day pass.
Use promo code (Exp: 4/22): EARTHDAY19
You need to claim the free pass today while supplies last. Your 24 hour pass will start from when you pick up your first bike ride.
A day pass is normally $12. You can take as many rides as you want while your pass is active, and the first 30 minutes of each ride is included. If you keep a bike out for longer than 30 minutes at a time, you will incur additional usage fees.