The parade will begin around 8 or 9 am, but if you get to the starting point early, you'll have lots of opportunities to take portraits of the participants while they're waiting for things to get started.
They form up from the Nawarat Bridge to the Train Station.  You should get to the bridge at least by 7 am if you wish to maximize your photo ops.  The lighting should be just right for portraits at this time.
After the parade, Suan Buak Hat Park should be the central point for the other festival activities.