"Neither House, during the Session of Congress, shall, without the Consent of the other, adjourn for more than three days, nor to any other Place than that in which the two Houses shall be sitting."
From: Obama’s recess appointments are unconstitutional
Yes, some prior recess appointments have been politically unpopular, and a few have even raised legal questions. But never before has a president purported to make a “recess” appointment when the Senate is demonstrably not in recess. That is a constitutional abuse of a high order.
From: Is Obama's appointment of Cordray illegal?
While this type of recess appointment is a common action for presidents, this one is fuelling a lot of argument because the Senate was technically still in session. The GOP senators, fearing Mr. Obama would do this, had been holding pro-forma sessions (pretend sessions where the Senate is called into session but no work is done). Senate Democrats used the same tactic during President Bush's administration.
These "sessions" are held every third day because traditionally Congress has had to be out of session for at least that period of time before a president can make a recess appointment. Cordray's is the first such appointment during a Senate break of fewer than three days since 1949.
From: When Is A Recess Appointment Not Really A Recess Appointment?
But Obama is the first President to make a recess appointment during a pro forma session. And business groups are now threatening legal actions to block the appointment.