"Lousy Smarch weather!" - Homer Simpson, The Simpsons
It seems to be
human nature to want more time. We want to
live longer, have
more time for fun activities,
more time with loved ones, an
extra day off, or
a month dedicated to an extended 31-day orgasm. Unless the Earth changes its rate of rotation and/or revolution or we change the rate at which we measure the passage of time we aren't likely to get that extra time, but in the world of animation gaining an extra day, week, or month isn't all that uncommon.
The example of this occurance that is most often cited is the "Treehouse of Horror VI" episode of The Simpsons in which Marge Simpson introduces a flashback that takes place "... on the thirteenth hour of the thirteenth day of the thirteenth month," the thirteenth month being "Smarch", a twenty-eight day month that supposedly came into being as a result of a printing error on new Springfield Elementary School calendars. This moment is played more for laughs than it is a crucial part of the plot, however, and it is unlikely that an extra month was actually added to Springfield's reality. A similar gag occurs in the "If I'm Dyin', I'm Lyin'" episode of Family Guy in which Lois Griffin mentions a Dateline: Katilsday edition of the NBC news magazine show, adding that NBC invented an extra day - "Katilsday" - in which to add another night of Dateline to the schedule. Again, this is played for laughs and does not impact the plot of the show, although unlike the Smarch example it is implied that an extra day in each week now exists thanks to NBC.
Shifting behind the scenes to Futurama brings to mind the story of how, in the show's early days, the animation and production crew was running short of time to complete the first season's worth of episodes. Producer David X. Cohen had taken to printing up weekly production schedules and one day as a joke he added an eighth day to the week's schedule - "Blernsday" - which would allow for extra time to finish the show. This joke eventually gave rise to the name of the year 3000's favorite Earthican sport, Blernsball, and eventually worked its way into the show itself in Season Four's "Bender Should Not Be Allowed On TV" in which a Blernsday Night Blernsball in mentioned as being a weekly televised sports event. Also consider an episodic example of added time for Futurama: the events in Season Four's "Crimes of the Hot" in which Professor Farnsworth changes the distance between the Earth and the sun, resulting in an extra week in each year and cooler climates on Earth. President Richard Nixon proclaims the extra time "Robot Party Week", incidentially.
References:
Futurama Season One DVD