Spooktober is a slang term for October, which is the month of Halloween, a holiday which involves dressing up in costume. Traditionally, companies and television channels will use the month to show horror-themed content. In the early 2010s, people began referring to the month as "Spooktober," a portmanteau of "spooky" and "October," to celebrate the month and the horror-themed media that comes with it.
While its unclear where exactly the phrase originated, it is possible that October Disney Channel specials from the mid-2000s had been misremembered as "Spooktobers." In the mid-2000s, the Disney Channel used the month of October to play original Halloween-themed movies, as well as other children's films around the holiday. These ran from 2005-2008. However, they were known as "Hauntober," Hauntober Fest," "Halloween Month," and "Wiz-tober," respectively. As early as 2012, tweets referring to Disney Channel's Halloween specials referred to the events as "Spooktober" (examples shown below).
The term began seeing more use the following year. That year, some of the first popular videos with the phrase began appearing on YouTube as makeup tutorials and Halloween-themed decoration videos (shown below).
In the following years, the terms expanded to month-long themed content on YouTube channels. For example, the Let's Play vash12349 uploaded a playthrough of the video game Until Dawn under the name "Spooktober" in 2015 (shown below, left). That same year, YouTube channel SmellyBellyTV gained over 3 million views with a Halloween-themed video of their family (shown below, right).
Later, the term was used more humorously and paired with intentionally non-frightening things. On October 2nd, 2016, Cyranek posted a mashup of Spooky Scary Skeletons with a Grand Illusions clip under the name "Spooktober," gaining over 111,000 views (shown below, left). The following year, channel Cabvno posted a Simpsonwave under the name Spooktober, gaining over 19,000 views (shown below, right). Popular YouTube Tyler Oakley ran several videos under the name "Spooktober" in 2017.
On Reddit, the term has been used in image macros about the popularity of Halloween-themed memes during the month. For example, on October 1st, 2018, a post about the upcoming "Spooktober" memes gained over 2,900 points in /r/dankmemes (shown below, left). Another post making a fake petition for updoots and downdoots in "Spooktober" posted September 29th, 2018 gained over 15,000 points (shown below, right).