According to the Citibank’s client manual (see the last paragraph on page 23),

Withdrawal Notice
We reserve the right to require seven (7) days advance notice before permitting a withdrawal from all checking, savings and money market accounts. We currently do not exercise this right and have not exercised it in the past.

After receiving a lot of attention online, Citibank confirmed the notice with the following comment:

When Citibank moved to unlimited FDIC coverage in 2009, we had to reclassify many checking accounts to allow for immediate withdrawals in order to ensure all customers qualified for the additional coverage. When we moved back to standard FDIC coverage with most major banks in 2010, Citibank decided to reclassify those accounts back to make them eligible again for promotional incentives. To do so, Federal Reserve Reg D requires these accounts, called NOW accounts, to reserve the right to require a 7-day notice of withdrawal. We recently communicated this technical requirement to our customers. However, we have never exercised this right and have no plans to do so in the future.

While Citibank claims to have no plans to enforce this rule, its good to know that it exists.