I've seen people writing : "Have an Octotastic day!". I've tried to search online but no useful results. From the context I feel its 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").