RIOT made a statement on Harrowing already.

Here's a link to the Red Post collection that has this statement. http://www.surrenderat20.net/2015/10/red-post-collection-one-for-all-live.html

Allow me to quote Ant In Oz on this one. "In Runeterra, the Harrowing can strike any day of the year, not just on a certain night/month, so only being able to tell Harrowing stories in October felt a little restrictive. Having said that, we are still going to be releasing a Harrowing story this month... but the next one could come at any time."