Every August, the two chambers of Congress adjourn until after Labor Day.
But why does Congress get such a long recess, and what actually happens—or doesn’t happen—during this period?
By the early 20th century, legislative sessions had become longer and longer, and Congress met for the majority of the year. Sessions hit a record length in 1963.
According to the Senate, that year’s session met from January to December with no break longer than a three-day weekend. By 1970, Congress mandated an adjournment as part of the Legislative Reorganization Act.
The law stipulates that “unless otherwise provided by the Congress,” both Houses must adjourn no later than July 31 each year or the first Friday in August that occurs 30 days before the first Monday in September, until the second day after Labor Day. The only exception is if the U.S. is in “a state of war.” Such a declaration hasn’t happened since 1941.
Sen. Gale McGee championed the idea of an August recess and the first official recess began in August of 1971. Since then, the recess has only been effectively canceled on one occasion, in 1994. Both chambers remained in session that year until late August.
Members of Congress also have the option to delay the start of the recess or call either chamber back into session. In some instances, Congress has had to return mid-recess, such as in 2005, when members returned to pass legislation to aid Hurricane Katrina victims.
So what do they do while on recess?
Recess doesn’t necessarily entail not working. While the Senate website says it’s “a chance for senators to spend time with family, meet with constituents in their home states, and catch up on summer reading,” some lawmakers use the time to campaign or raise money if they’re up for reelection, or visit offices or hold town halls. Offices are also still open to receive mail and take calls from constituents, but there is no activity on the House or Senate floor throughout the month.
There is one other thing that can happen during a recess: a president can execute a pocket-veto or make recess appointments.
This fact, along with a desire to disrupt the schedules of members of the opposing party, has set off a fair share of political maneuvering in recent years. Particularly when the chambers have been divided—or the majority is simply not from the same party as the sitting president—some members have taken advantage of a rule that prevents a full recess from taking place.
An adjournment cannot take place if both chambers do not consent to it. The Constitution states that neither chamber can adjourn for more than three days without consent from the other, forcing Congress to meet, at least in a pro forma session, every three days until the formal end of the recess.
A pro forma session does not require full attendance, nor does much legislative action take place. Typically, someone simply goes to the House or Senate floor and gavels in and out.
In 2017, for example, Senate Majority Leader Mitch McConnell cut the first two weeks of August recess in an attempt to get Republicans to repeal the Affordable Care Act. He similarly tried to cancel last year’s summer recess for the Senate—largely to hinder Senate Democrats from going home to campaign ahead of the November election. The Senate ended up convening for a total of eight days over the course of the adjournment period.
When serving as speaker of the House the first time, Rep. Nancy Pelosi also called the House back into session for an afternoon in 2010.
This year, Democrats are trying to compel the Senate back into session in the wake of several mass shootings in recent weeks, to consider legislation related to firearms, including a background check bill that was passed earlier this year by the House. Six top Democrats held a press conference in the Capitol this week in an attempt to pressure the Senate Republican leadership.
“The time is not simply for reflection. The time is not for a moment of silence,” House Majority Leader Steny Hoyer said. “The time for the Senate is to act. The time is to listen to the American people.”
Hoyer further said that the House judiciary panel will return early from their recess to consider new “red flag” legislation in hopes of putting additional pressure on Republicans.
Thus far, Senate Majority Leader Mitch McConnell has refused to take up the bill despite widespread, bipartisan support for it.
McConnell said in a radio interview last week that if he did call the Senate back from recess early, “we would just have people scoring points and nothing would happen. There has to be a bipartisan discussion here of what we can agree on. If we do it prematurely it will just be another frustrating experience."
More must-read stories from Fortune:
—When will Twitter ban white nationalists? Civil rights leaders urge action
—Several states suing Trump's EPA over new 'dirty power' rule
—Why the U.S. labeled China a currency manipulator
—How Trump’s plan to import Canadian drugs would work
—Listen to our audio briefing, Fortune 500 Daily
Get up to speed on your morning commute with Fortune’s CEO Daily newsletter.