A session begins when a chamber convenes , or assembles, and ends when it adjourns . A calendar day on which a chamber convenes and then adjourns or recesses until a later calendar day may be called a "calendar day of session" or, more informally, simply a "day of session" for that chamber. A legislative day ends only when the chamber adjourns, and a new legislative day begins whenever the chamber reconvenes after an adjournment. By recessing overnight rather than adjourning, a chamber may continue a single legislative day into a second calendar day. Conversely, instead of taking a short recess within a calendar day, a chamber may adjourn briefly and reconvene later on the same day. If, however, the President were to call an extraordinary session after the sine die adjournment of one regular annual session and before the convening of the next, the presidentially called session would be a separate session of Congress, additional to the regular annual sessions that convene pursuant to the Constitution, and would receive a separate number within the regular sequence. As with sine die adjournments of a session, each chamber normally provides permission for the other to adjourn for three days or more by means of a concurrent resolution, adopted by both. A "recess of the session" (for more than three days, pursuant to a concurrent resolution) does not terminate an annual session, but only puts it in a state of suspension, just as a recess of the daily session does not terminate the legislative day, but only puts it into a state of suspension. Pro Forma Sessions

In the primary sense of the term, a pro forma session is considered to include any daily session which is held chiefly to prevent the occurrence of a "recess of the session" (that is, an adjournment for more than three days within an annual session) or forestalls a sine die adjournment. Nevertheless, no adjournment for three days or more within the session occurs, and no adjournment resolution is required. During a recess of (the annual session of) the Senate, nominations retain their pendency, except that, if the recess is longer than 30 days, they are returned to the President unless the Senate otherwise orders. In recent times, when Congress has met in lame duck session, the post-election portion of the annual session has most often been separated from the pre-election portion by a recess of the session. "Recess" and "Session" are clearly used here in a sense pertinent to the annual session, not the daily; the President could hardly avoid asking the Senate to confirm his nominations just by making appointments during a recess in the course of a daily session, or even during an overnight or weekend recess within the same legislative day. On the other hand, it has long been the general practice that for purposes of this clause, a "recess" can be either an intersession recess or an intrasession recess; that is, either a period of sine die adjournment or a recess of the session, as described earlier in the sections on " Adjournment Sine Die " and "Recess of the Session ." In practice, however, Presidents until very recently appear to have made recess appointments during intrasession recesses of the Senate only when the recess would be at least 10 days long. Based on the Court's rationale in Noel Canning , the Senate may effectively utilize pro forma sessions to prevent the President from making recess appointments. In this way, pro forma sessions tend to make days of continuous session elapse as fast as calendar days. If a period measured in calendar days begins on the same day for both chambers, it will necessarily also end on the same day for both. In the second session of the 110 th Congress, for example, if an action period of 60 days of session began on the day the annual session convened, it would have started in both chambers on January 3, 2008, but the 60 th day of session would have occurred in the House on April 21, and in the Senate not until May 16. To forestall this situation, many statutes that count action periods in this way stipulate that the expedited procedure is available during the "first period of [the stipulated number of] days of continuous session of Congress beginning after" the designated initiating event, and also provide that "continuity of session of Congress is broken … by an adjournment sine die" (or, sometimes, only by a final adjournment sine die of the Congress).
The House and Senate use the terms session, adjournment, and recess in both informal and more formal ways, but the concepts apply in parallel ways to both the daily and the annual activities of Congress. A session begins when the chamber convenes and ends when it adjourns. A recess, by contrast, does not terminate a session, but only suspends it temporarily.
In context of the daily activities of Congress, any calendar day on which a chamber is in session may be called a (calendar) "day of session." A legislative day, by contrast, continues until the chamber adjourns. A session that continues into a second calendar day without adjourning still constitutes only one legislative day, but if a chamber adjourns, then reconvenes later on the same day, the single day of session includes two legislative days. Conversely, if a chamber recesses and then reconvenes on the same day, the same day of session and the same legislative day both continue. Finally, when a chamber recesses overnight, instead of adjourning, although a new calendar day of session begins when it reconvenes, the same legislative day continues.
A regular annual session of Congress begins when the two chambers convene in January, pursuant to the Constitution (or to law). An annual session ends with an adjournment sine die. Until the next annual session convenes, Congress is then in a period of sine die adjournment (or "intersession recess"). If the President were to call an additional, "extraordinary" session, it would be procedurally similar to a regular annual session.
The Constitution provides that neither chamber may adjourn for three days or more without the consent of the other. The two houses consent to each other's sine die adjournment by adopting a concurrent resolution, called an "adjournment resolution." They use a similar vehicle to allow each other to suspend their daily sessions for three days or more without terminating their annual session. Such a suspension is called a "recess of the session," an intrasession recess, or, more formally, an "adjournment for more than three days" within a session. To avoid the need for a concurrent resolution, a chamber may hold pro forma sessions on such a schedule that no break of three days or more occurs.
Legislation retains its status, and may continue to receive action, until the last session of a Congress adjourns sine die. Nowadays, measures are "pocket vetoed" only when unsigned by the President after a final adjournment sine die. Nominations, by contrast, will be returned to the President if they remain pending whenever the Senate adjourns sine die or recesses its session for more than 30 days, unless the body otherwise orders.
"Lame duck sessions" are periods when Congress is in session after election day, but before the newly elected Congress takes office. Nowadays, they are not separate annual sessions, but portions of the last regular annual session of a Congress, usually separated from the pre-election portion by a recess of the session or by a period of pro forma sessions.
Recent Presidents have made recess appointments during intersession recesses (periods of sine die adjournment), even very short ones, but have usually done so during intrasession recesses only of 10 days or more. Pro forma sessions have sometimes been used to preclude recess appointments by preventing a recess of the session. In 2014, the U.S. Supreme Court held that the President may make recess appointments during recesses of 10 or more days, but the Senate may utilize pro forma sessions to effectively prevent such appointment opportunities.
Certain statutes provide that Congress may disapprove, or must approve, specified actions of the executive branch by using expedited ("fast track") procedures during specified periods. These periods may be defined in calendar days, days of session, legislative days, or "days of continuous session." Days of continuous session include all calendar days except those on which either chamber is in a "recess of the session."