Hey all! I wanted to make a (revised and updated) dedicated thread where people could share any questions they might have about the ongoing story challenges.
Pretty much, if you’re unsure how the challenges work, or have questions on any of the challenge rules or themes—or if there’s anything about the process you could use clarity on—just ask your question here! And I’ll do my best to answer it. 
Also, I've added an FAQ below! This FAQ used to live on my old Philip K author profile. But Martin pointed out that no one really saw it there. 😅 So I'm putting it here instead. Feel free to ask for clarification on any of these. And I'll update the FAQ as needed!
Q: When will the next challenge be?
A: Currently I’m trying out a format where there are three challenges per year—one in late-winter, one in early-summer, and one in autumn. The winter and summer challenges are the themed challenges. And those are the ones where you get to vote on the theme! The autumn challenge is the recurring Halloween challenge, which has slightly different rules than the usual challenges.
Q: What will the theme be for the next themed challenge?
A: I like to give users the chance to vote on a slate of potential themes! So ahead of each challenge, I craft a list of five potential themes, and then the users vote for their favorite. When it’s time to vote, there’ll be an announcement on the site’s front page with a link to the poll. You’ll have a week to cast your vote. The challenges themselves open for submissions a month after the vote has concluded. This gives people time to think of a story idea that both inspires them and fits the theme.
Popular themes from previous voting slates which didn’t win their round might pop up again later, but not right away.
Q: How are the Halloween challenges different from the regular (themed) challenges?
A: Two things are different: authors are not anonymized like in the regular challenges, and authors can also enter more than one story into the Halloween challenge.
Q: Can you enter multi-chapter stories into the story challenges?
A: Absolutely! Thanks to a neat update that Martin coded, you can now enter multi-chapter stories into the story challenges! They used to only be permitted in the Halloween challenges, but this is no longer the case.