I don't have the full details, so I don't know if this is just for people in the U.S. (I think it might be), or has other restrictions, but here's the content from the email I received from Ancestry.com:
This weekend only, we're giving you free access to more than 1 billion UK records—so mark your calendars and get ready to uncover some amazing family gems.
*Access to the records in the featured collections will be free from February 17, 2017 at 2:00 p.m. ET to February 20, 2017 at 11:59 p.m. ET. After the free access period ends, you will only be able to view the records in the featured collections using an Ancestry World Explorer or All Access paid membership. To see a full list of the records in the featured collections, please click here.