WHEN: Double ISO-8 events?

Marvel Puzzle Quest MPQ Events, Tournaments, and Missions
1

Only been playing sine October. When I started there was this double ISO-8 thing going on due to the anniversary. Also around Christmas time we got double ISO-8.
Now, I’m wondering; how often can I expect these events to occur? On which occasions except the X-mas holidays and anniversaries (if any at all)?

Thanks.