TV Hiatus explained: Why your favorite shows take breaks during the seasonAdd to Favorites | Glee
Throughout the television season, primetime shows frequently take breaks -- an episode here, two or three episodes there. It's particularly common in the winter months around the holidays for shows to take an extended break, leaving reruns in their place. This is called hiatus.
The most simple answer is: math. There are about 22 episodes in a typical primetime season on the five major networks. There are 52 weeks in a year. Additionally, it takes more than one week to shoot an episode -- your average episode of, say, "The Vampire Diaries" or "Revenge" takes 8 days to shoot, and that's not counting writing, prep, and post-production. If there were no hiatuses, your season would begin in late September and end at the end of January, and then you'd have to wait 8 months to discover how that cliffhanger turns out.
As with most things in the television industry, what it really comes down to is money. With the exception of premium cable networks, most networks make money off of their television shows by selling advertising. The advertising companies use ratings to decide how much a 30-second commercial is worth during a certain show, based on how many eyes will actually see that commercial.
Though ratings information is collected every week, Nielsen is more thorough in their ratings investigation during "sweeps" periods, which fall roughly in November, February, May, and July. During these four-week periods, Nielsen asks several panels of homes to keep a detailed paper diary of the television they watch, live and via DVR. They analyze what certain demographics are tuned into.
Generally, it's in the networks' best interests to make sure their best, most attention-grabbing episodes air during sweeps, because then more Nielsen families will watch, then the networks can sell commercials for more money. That's why you'll most often see big-name guest stars or shocking twists during these periods. Shows often go on hiatus right before these breaks so that the networks can save their best episodes for sweeps weeks. Shows return from hiatus with a big promotional push and lots of press right as the advertising companies start paying attention.
On the other hand, sweeps are when competition amongst the networks is at its highest, and sometimes, it's in a network's best interest to avoid that. In 2011, "The Vampire Diaries" was on hiatus for half of November sweeps; one possible explanation for this scheduling choice is that with other networks putting out their best material, The CW was better off bowing out than trying to fight outside its weight class.
There's a lot of criticism of the sweeps method (and of the ratings system in general). Many experts also feel that the 22-episode season isn't ideal and that year could be broken up more efficiently. That's all up for debate. One thing all experts agree on: it's like, a total bummer when you expect a new episode and tune in to a rerun.
There are other reasons for hiatus. Sometimes complications with production force a show into hiatus. Maybe the writers need time to catch up or to regroup for a new creative approach. Maybe a star has a meltdown, like "Two and a Half Men" when Charlie Sheen was at his winningest. This season, "Bones" went on hiatus to accommodate its star's maternity leave. Over the 2007-2008 season, many shows were forced into hiatus by the Writers Guild of America strike.
But as a general rule, if you turn on your television only to discover that there's a rerun on, it's because of two things: Math, and money.