From Special to Series

Series that began life as prime-time specials, sometimes related to holidays. This seems to particularly be the case with adaptations. Seemed to be very popular in The Eighties.

In Japan, there have been some cases of Original Video Animations being picked up to become broadcast series, including You're Under Arrest and Black★Rock Shooter.

Compare Poorly-Disguised Pilot, Five-Episode Pilot, and Summer Replacement Series. Sometimes overlaps with Pilot Movie.

Examples of From Special to Series include:

Anime and Manga

You're Under Arrest had a four-OAV run before being picked up as a broadcast anime.
Black★Rock Shooter.
The One Piece Christmas special was originally supposed to be a two-episode filler where the cast is in feudal Japan and Luffy is a detective, but later evolved into reoccurring filler episodes. They were easy to plot, genuinely funny, could be fit in at any time, and brought back a lot of old characters in as cameos.
- Recently,^[when?] Chopperman was given his own series. It began as a tiny short, then turned into some successful merchandise, and now it stands on its own two feet.

Live-Action TV

The Adventures of Pete and Pete started out as a series of shorts during commercial periods, followed by full-length specials, then the regular series.
The Sarah Jane Adventures began with a New Year's special.
Bo Selecta, or at least lead character Avid Merrion, started as filler shorts during adbreaks for one season of Big Brother Live.
This is very much the purpose behind Comedy Lab.
The Australian show Good News Week was canceled back in 2000. In 2008, a reunion special was planned, but the Writer's Union Strike of America caused Channel Ten to revive it fully.
Another Aussie example, Hey Hey It's Saturday, was a variety show that was on air for nearly 30 years, starting as a children's program and eventually becoming a prime time variety show, in 1999 the show was wrapped up, but in 2009, 2 one off reunion specials were aired on consecutive Wednesdays. These specials were a runaway hit and this resulted in the show now being back on air every Wednesday.
America's Funniest Home Videos began as an hour-long special hosted by Bob Saget and Kellie Martin in November 1989; it was brought back as a weekly series two months later with Saget in tow, and has aired ever since with two host changes since then (with the minor exception of a period from the fall of 1998 to the summer of 2001, when the show was reduced to a series of specials).

Western Animation

A Charlie Brown Christmas, It's the Great Pumpkin, Charlie Brown and the other Peanuts specials in that sequence eventually led to The Charlie Brown and Snoopy Show. This one is a case where the specials are currently more famous than the show they launched.
Garfield had several specials under his belt before Garfield and Friends.
G.I. Joe had two mini-series before the actual series began.
My Little Pony
The Simpsons: "Simpsons Roasting on an Open Fire" aka "The Simpsons Christmas Special" was always intended to be just one more episode of the show, but the fact that Fox chose to air it as the first episode is probably due to the influence of this trope.
Similar to the Simpsons example, the 2008 George of the Jungle series started with a Christmas special.
All Grown Up! was originally a special, but was popular enough that they made it a successor series to Rugrats.
Strawberry Shortcake was originally just a series of specials. When she was revived in the 21st century, she got a proper TV series.
The Raccoons originally had four specials, the first about Christmas.
Madeline (the animated series) started as a series of TV specials in the late 1980s-early 1990s. She had her own TV series from 1993 to 2001.
Transformers Generation 1: The original cartoon started out as a five-part serial.