Of the 13,000-plus days since Jan. 1, 1978, both chambers of Congress have been in session at the same time for about 4,700 of them -- about a third of the total time and a little fewer than half of all weekdays. The Senate has worked more than the House, having been in session about 42 percent of the time to the House's 39 percent.

A look at the the past 37 years of Congressional activity, as far back as online records from the Library of Congress go for both chambers, reveals that your likely stereotypes about the amount of time Congress spends doing the people's work is probably about right.

For example, during the 1,917 weeks since the start of 1978 (which conveniently fell on a Sunday), the Senate has worked a full week -- Monday through Friday -- 601 times. The House has done that far less, at 362 times. And if you're looking for weeks in which both the House and Senate did full five-day week, you're talking about 258 times -- 13.5 percent of the time.

There are two reasons for this. The first is that Congress takes a lot of time off from legislating. The graph below shows how frequently Congress worked on any given day of the year. Notice the lulls in August, in December, in January, at the end of May. Those are periods when Congress usually isn't in D.C,, much less in session.

Curious about a particular day? We broke out the percentage of time that Congress was in session by day. Check your birthday; check your anniversary. See how often Congress has worked that day since 1978.

The other reason is that Congress likes to take Fridays off. Both chambers of Congress have been in session less than a quarter of the time on Fridays since 1978. The Senate does slightly better here, having been in session 44.5 percent of the time on the day before the weekend.

If you're hoping that the 113th Congress might still redeem its apparently inevitable future as the least productive in modern history, some bad news: Historically, October has been one of the more productive months for Congress -- but only when it it's not an election year.

The longest stretch both chambers have been in session simultaneously was 13 days, ending on Oct. 12, 1990. The longest stretch in which at least one chamber was working was 31, coinciding with the Clinton administration shutdown. The longest period neither was doing anything? Several months at the end of the 104th Congress.

An obligatory caveat: Being a member of Congress is not only about being on the floor in the Capitol, debating policy and casting votes. Members of Congress often hold constituent events on weekends and, occasionally, they're in session then, too.

But it is hard to escape the implications of Friday being the weekday on which the House and Senate are least commonly in session. Perhaps it's because they've gotten everything done between Monday and Thursday. Perhaps it's something else.