@AllMenAreIslands I was happy to use the week to make progress on Atlantis rather than buying an event. I got the locks needed for my last lighter side and my first darker side before Christmas 2018 started up.
BlameTheParrot I don't see any "Christmas Cars" in Christmas 2015 (nor in 2017). Did you mean reindeer ornaments? I'm a bit past midway in Atlantis and the reindeer ornaments are still in my daytime camp - probably not much longer as they are about the lowest regen when I tweak it to 1 hour refresh, but they've had a good run.
The stone was handy in China when I hadn't finished unlocking the grid.
Chani If you haven't played it yet, the first run through returns even more gems because of the gems from completing achievement levels. I didn't keep track of how many, but it's probably enough to make it free to play or net some gems.