There are apparently going to be three in total, each collectable over two days (all three are weekends) and the information to find what is required is being emailed to premium members on a Friday (i.e. 8 days before the relevant weekend) and 'made available' to everyone else at the beginning of that weekend itself. The next two relevant weekends appear to be the 13/14 August and 27/28 August. The requirement for the weekend of 30/31 July was to find a cache with 10+ favourite points.
We did solve the puzzle in advance for the last one, which was not difficult once you knew what was required, and there were a lot of hints posted on Groundspeak forum. But we can imagine that people who don't like puzzles may have been put off by having to solve one. We don't know whether the puzzle was just for premium members in advance or was also included in the information 'made availble' on the weekend itself as we didn't see that. This information will be apparently posted on Geocaching.com, Geocaching® app, Facebook and Twitter). Does anyone know?