Two years ago they released the PB just prior to July 4th.
I only remember because I was so excited to try out new mapping features on a very long drive I was taking prior to the holiday.
Did a bit of digging... last three public beta release dates were:
iOS10 public beta July 7th
iOS11 public beta June 26th
iOS12 public beta June 25th.
My guess is that Apple specifically mentioned the July time-frame in order to set expectations. The week of the 8th is easily the most likely release window.