Anywhere with a good view of the south-facing (Hub-facing) facade of the castle.
That would be somewhere in the Hub (or adjacent to the Hub,) or on the northern half of Main Street.
When to arrive is dependent on the overall crowd level in MK.
I'd guess 45-60 minutes before the show... but, since on many nights the MSEP will ALSO be running, the areas i noted will likely fill with guests even much earlier that.