It is rare that Blizzard announces PTR start time. It tends to be in the afternoon Pacific time, but the actual hour varies.
Keep in mind that the PTR is a test environment and is shared globally, so it can be pretty unstable (test builds), and very crowded. Oh, and remember that if you copy your Live account data over, it replaces any data you have on the PTR, so be sure to do it before creating a Season character.