Formal congressional sessions occur twice in any two-year inter-convening period; there is one session per year. During this time, Congress is obligated to conduct all of its legislative, judicial and executive duties, as outlined and authorized by the Constitution. However, there are a variety of different forms of session, each with its own character.
The most common session is the daily one, in which Congress goes about its daily duties. Special sessions occur when Congress meets after the formal adjourning of a one-year session, whereas extraordinary sessions occur when the president recalls Congress to meet during recess or after adjournment. Joint sessions require both houses to meet at once, as during a presidential address or for the counting of ballots after a vote. Closed sessions, on the other hand, are held secretively, usually to discuss sensitive or classified information that falls under Congress' purview. For example, a closed session may be held to discuss issues regarding possible impeachment, matters of national security, or other circumstances and sources identified as confidential. Executive sessions typically follow presidential nominations or debates related to pending treaties. Historically, executive sessions were held in secret, as closed sessions, though this business was commonly made public by the beginning of the twenty-first century. Finally, there are what are pejoratively called "lame-duck" sessions, where Congress returns after November elections, usually with many members conducting the last of their business after having been voted out of office. Thus, these members are considered "lame ducks," as they have only limited time remaining and little pull left over proceedings before their replacement.