From senate.gov
pro forma session - A brief meeting (sometimes only several seconds) of the Senate in which no business is conducted. It is held usually to satisfy the constitutional obligation that neither chamber can adjourn for more than three days without the consent of the other.
Which, based on their own definition, is required since they aren't on formal recess.
Hell, its a published schedule for every three days.( again, senate.gov ).
FRIDAY, JANUARY 6, 2012
The Senate will convene at 11:00 AM and will conduct a Pro Forma Session.
The Senate will then conduct Pro Forma Sessions on the following dates:
Tuesday, January 10, 2012 at 11:00 AM.
Friday, January 13, 2012 at 12:00 PM.
Tuesday, January 17, 2012 at 10:15 AM.
Friday, January 20, 2012 at 2:00 PM.
The Senate will next convene for business on Monday, January 23, 2012 at 2:00 PM. There will be in a period of Morning Business for two hours. At 4:00 PM, the Senate will proceed to an Executive Session to consider the nomination of Cal. #438 (John Gerrard to United States District Judge for the District of Nebraska). There will 90 minutes of debate on the nomination followed by a vote on confirmation at 5:30 PM. On Tuesday, January 24, 2012 at 2:15 PM, the Senate will have a cloture vote on the Motion to Proceed to S. 968 ( Preventing Real Online Threats to Economic Creativity and Theft of Intellectual Property Act of 2011).