The reason? In an effort to block President Obama from making recess appointments—which the Constitution allows presidents to do—Congressional Republicans have kept Congress technically in session.
The Washington Examiner explains :
The Republican-controlled House used a procedural move to help force this issue. Though it's the Senate that must confirm presidential appointments, under the U.S. Constitution, it cannot adjourn for more than three days without the approval of the House.
So, instead of adjourning, both the House and Senate will be conducting what are known as “pro forma” sessions. What that entails, essentially, is having a member of Congress stick around the Capitol to strike the gavel for what are sometimes seconds-long sessions , according to the Examiner.
And if history is our guide, it seems Congress will have to hold these perfunctory sessions at least once every three days.
According to this 2010 Congressional Research Service report  [PDF], the Constitution doesn’t actually say how long the Senate must be in recess before the president may make a recess appointment, but in 1993, the Justice Department suggested that the number was three days. According to Roll Call, the Senate will be holding pro forma sessions nine times , even while most of its members are back at home.
Lest you think this maneuver is specific to Republicans: Democratic Senate Majority Leader Harry Reid also used the strategy under President Bush.
Republicans have used it frequently under President Obama—during the Memorial Day recess , the July 4 recess  and through the rest of this month. In June, the freshman class of House Republicans sent a letter to House Speaker John Boehner urging him to prevent recess appointments  [PDF] and offering their services for covering the pro forma sessions.
“We understand that our request will very likely mean that the House of Representatives will meet no less than once every three days for the remainder of 2011 and all of 2012,” the lawmakers wrote. “We stand ready to assist you in ensuring there are always sufficient members to cover the necessary pro forma sessions.”
By keeping Congress technically in session, Republicans will be able to keep the president from naming a temporary head to the recently formed Consumer Financial Protection Bureau. The new bureau's powers are limited without a director, and Republicans have vowed to oppose the confirmation of a new director unless the agency's authority is rolled back . They've speculated  that President Obama might resort to a recess appointment in order to get his new nominee, former Ohio Attorney General Richard Cordray, finally on the job.
Overall, President Obama has made relatively few recess appointments. The New York Times noted  that he’s made 15—which the White House openly announced last year , citing "Republican obstruction"—compared to President Bush’s 171 and President Clinton’s 139 recess appointments.