Per the Citi Bike Tumblr:
We’ll be taking Citi Bike offline at 10 pm Friday through the weekend for maintenance. After 10 pm you will not be able to check out a bike.
During this time members will not be able to access their accounts. The membership sign-up page, Citi Bike station map and Citi Bike app will also be unavailable.
We’ve timed this work to make sure we’re back online in time to get our members pedaling by Monday morning.
We will be extend all active Citi Bike memberships by three days and issuing refunds for anyone who purchased a 24-hour pass on Friday 3/27. We will also offer refunds to anyone who purchased a 7-day pass earlier in the week.
We apologize for any inconvenience this might cause. We wanted to make these upgrades to improve Citi Bike before the nicest riding weather begins.
Updated 3-28 5:56 p.m.
Good news! We're coming back online a bit early. Find out more: http://t.co/utD1AVN1rY— Citi Bike (@CitiBikeNYC) March 28, 2015