Just so everyone knows, this isn't a push notification. I can say this with certainty, because my iPad has not been used within the last 72 hours, and earlier today in the car when I unlocked it to jot something down in Notes, I received the pop up. I didn't have a wifi connection in the car, and it's a wifi only iPad. This is very clearly a hard coded time/date notification, as most have already speculated. The reasons why, we can only speculate.
Also, I restored my iPhone earlier because I've been having an issue with storage on the phone slowly depleting over the course of a few weeks with every version of this beta (iTunes will show 4gb free, phone will match at first, then over a few weeks will dwindle down to almost no free space available - despite iTunes showing the correct storage free) and after restoring and updating the PB, the pop up was the very first thing shown once the phone had fully restored from backup.