I should make a small note here ... thanks for the reminder Figgy.
I have amended my posts above with quotes from the Pottermore Insider.
The riddles are a part of the Harry Potter and the Half Blood Prince chapters. Each riddle must get answered before you are allowed into the moment(s). These (I don't believe) will be going away any time soon. Every day we are getting a new chapter; which is great. I believe it is Pottermore's way of bringing in a 'book' with some daily magic (vs opening a new book all at once). I am positive the chapters - with all the 'read abouts' and 'news from JKR' - will be there for a while.
This was posted on the Pottermore Insider:
Quote:
Posted by Pottermore Editor on Monday, December 08, 2014
Just to put everyone straight on the misleading media reports of ‘Twelve New Stories for Christmas’. As true Potter fans worked out straight away, our 12 days of festive surprises and accompanying rhyming riddles refer to new potions and Moments in addition to several new entries by J.K. Rowling that will be posted on pottermore.com between 12th and 23rd December. Sorry to disappoint fans, but there will not be a set of 12 new short stories, one for each day of Christmas. Even Pottermore wouldn't be that generous.