Wednesday, Thursday, Friday, Saturday, or Sunday.
Sunday is the least likely because timezones causing some countries to be almost a day ahead of others meaning that it'll already be August for parts of the world. Saturday is plausible but I recall hearing that Playtonic Games doesn't typically work on weekends and I certainly don't ever recall getting an update on a Saturday. That said, I predict that we're getting the codes Friday at the latest unless someone at Playtonic wants to put in overtime in which case it would be Saturday.
But if they plan on surprising us I don't think it'll be last minute because then we will have ruled every other day out... This is starting to look just like the Unexpected Hanging Paradox (watch the video below if you don't know what I'm talking about).
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