For events like this, it can't be an automatic in-game update. It has to be downloaded from the store, presumably because of the size of the update and the approvals needed (i.e., from Apple). That being said, on iOS, your app store should automatically let you know when updates are available, so you should know fairly quickly. Pretty sure Google Play does the same thing, although I don't know what the turn around time is like. They can, and do, sometimes force users to update after a few days if they want to continue playing but that already leaves you a few days behind.
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