I've seen people writing: "Have an Octotastic day!". I've tried to search online, but I didn't get any useful results. From the context I feel it's a synonym for "fantastic". I couldn't see any details on usage of that word. Is it something made-up?
At MrHen's kind suggestion, I'm submitting my earlier comment as an answer...
It's very common in informal writing for authors to add "-tastic" to words to make them sound more effusive. So it's not that October is especially fantastic, but merely that you didn't happen to encounter an occurrence of "Maytastic" or "Junetastic" when their time rolled around. Having said that, I will admit that some months lend themselves to portmanteauing more readily than others. I doubt, for example, that you'll run into an occurrence of "Februtastic," even though February is a truly outstanding month.
As Jim notes in one of his comments above, the suffix "-rific" (borrowed from "terrific") has a similar tendency to appear as a would-be excitement enhancer in a multitude of settings, as in "lobsterrific" or "ennuirific." Yet another such suffix (somewhat less commonly used) is "-tacular" (borrowed from "spectacular").
Belated acknowledgment: It seems pretty clear to me now that, with regard to the question of where octastic comes from, Bryan's answer is the correct one. I'll keep my answer here live because I think it still has some value in connection with interpreting other words with suffixes like -tastic; but in the specific instance of octastic, Bryan undoubtedly has it right.
Yes, it is from GitHub.
It comes from their logo which is an Octocat.
The origin of the term comes from Github and his mascot, the octocat. The word acrually is a combination of octopus and fantastic, such words are called portmanteaus.
It appears to be an invented term to increase the awareness about science, octopuses in particular. It would increase our interest in the marine wildlife, but until now it has not reached its purpose...
The actual meaning doesn't exist, but in the broader sense it could mean "have a fantastic day".