After the half season tickets finished I think, so mid January I think, though that could have been because they were a new idea, I'd have hoped they'd have been put out at the start of the season because personal I cant make many matches due to work etc so these were a brilliant idea for people like me who can only make limited amount of matches
Last year they were announced 6th October: https://www.barnsleyfc.co.uk/news/2016/october/flexi-ticket-membership-scheme/
The first game I could have gone to this season is the Villa game. I won't be doing because there isn't a flexi ticket and I'm not paying £30.