Sunday: few people looking to start something new, languish for a few days.
Monday: it's *Monday*.
Tuesday: Still beginning of the week, things are still being piled on
Wednesday: nothing wrong with it
Thursday: early adopters try, advantage of full Friday + an extra day of users before weekend
Friday: people finish / wind down / have more free time to start things
Saturday: In the swing of X, or looking to start X.
My final take from that: Thursday gives you part of a day with early birds to find game-breakers ASAP, and Friday and Saturday are useful days to have early to boost your numbers at the start. You're also sure to get all of Friday by starting on the day before. Wednesday gives little advantage over Thursday, but costs you another day before bug-fixing weekend / next week. And early week releases are, well, too close to Monday. Nobody likes that.
All of which is wildly subjective, and is merely justification and not reason. But it makes some sense in this light (to me, at least).
The second part is the fact that Saturdays are the biggest App Store sales day of the week, so by launching on a Thursday, you have a couple days to build up steam and be high in the charts by the time Saturday rolls around.