Comic book publishers release comics on the same day each week. The same titles are released on the same week each month (except for issues that are late for one reason or another). This works well, except that every so often there's a month with five Wednesdays, or Mondays, or whatnot. Thus, was born the Fifth Week Event, wherein the publisher releases a line of one-shot comics to fill the empty space on the schedule.
Usually, the comics in a Fifth Week Event are all grouped around some theme. In 2006, Marvel did a series of "What If?" Elseworlds that all took place in the same universe. In 1998, DC did a series of "Girl Frenzy" comics that focused on various female supporting characters.
Fifth Week Events usually don't have a huge impact on continuity, since people who only read ongoing series won't see them. There are exceptions - one of the Girl Frenzy titles was used to kick off the series Young Justice.
Comic book publishers will frequently use Fifth Week Events to renew trademarks on character names. United States trademark law requires trademark holders to publish works using the trademark in order to renew it. As part of the Fifth Week Event, many characters who haven't recently held a regular series will have a one shot tie-in to the event with the character's name in the title alongside the name of the Fifth Week Event. For example, DC Comics's Tangent event had comics such as Tangent: Doom Patrol, Tangent: Metal Men, Tangent: Power Girl, and Tangent: Sea Devils.
Sometimes, sufficiently popular Fifth Week Events are revisited again later - the 2006 "What If?" was a return to a theme used in 2005 (which in turn was based off a cancelled ongoing series), and "The Justice Society Returns" led to an ongoing JSA series.
Of course, not everything that comes out on a fifth week is a Fifth Week Event. For a time, the closely-linked Superman titles (so close that they were often more like a single weekly series than four monthlies) had an extra series, Superman: The Man of Tomorrow, that they ran whenever a fifth week came by.
- "Silver Age" was a DC Fifth Week Event that celebrated the spirit of the Silver Age, bringing back whimsical concepts like Dial H For Hero and the Metal Men.
- "Marvel Mangaverse" redid various Marvel characters and stories in the style of manga. It was well enough received to spawn a (short-lived) monthly afterwards, and also to merit a 2006 New Mangaverse Revival miniseries.
- DC's "Tangent Comics".
- "Marvels Comics", a group of in-universe comics published about Marvel's superheroes, ranging from the completely approved (Captain America drawn by Steve Rogers) to the gratuitously inaccurate (Spider-Man, X-Men).
- DC's "Sins of Youth" event, which turned the adult superheroes into teenagers while aging the teenage heroes to adulthood. Lampshaded when an age-regressed Steel wondered why things like this happened "every fifth week or so." As a Fifth Week Event, it was unusual in that it actually grew out of an ongoing plotline in the Young Justice comic.
- Sins of Youth is also one of the few fifth-week events to actually keep comic book plotlines moving. For instance, Superboy learned Superman's secret identity, and Jack Knight passed on the Cosmic Rod to the Star-Spangled Kid after seeing her aged as "Starwoman". Shortly after that, she became Stargirl.
- The Justice Society Returns was intended to renew interest in the JSA. The event was a flashback story set in 1943, and the last page was a preview of the new JSA revival series that came out shortly after (and proved wildly popular).
- Amalgam Comics is sort of a Fifth Week Event, except it was published in week 4. Spinning out of the Marvel Vs. DC miniseries, and published the same month, it was released in between issues 3 and 4 to fill the extra week.
- The week of August 31st, 2011, marked the end of Flashpoint and the beginning of the New 52.