From the Pottermore Insider:
Pottermore has witnessed some frenzied brewing and duelling over the last few days and we’d like to congratulate Slytherin on winning the inaugural Pottermore House Cup.
Commiserations to the other houses, but your chance will come again as competition for the next Pottermore House Cup is already underway!
As part of their reward for winning the Pottermore House Cup, Slytherin will be able to explore the new chapters of Harry Potter and the Chamber of Secrets, twenty four hours ahead of the other houses.
Please be aware that only students who were in Slytherin when the inaugural Pottermore House Cup was awarded, will get early access.
For students who are not in Slytherin, don’t despair. The first chapters of Harry Potter and the Chamber of Secrets will be available for everyone immediately after this twenty four hour period.
From then on, all users will be able to explore the new chapters, discover new Moments, and continue their journey on Pottermore.
Congratulations to Slytherin House!