aussiegal,
The actual festival is on Tuesday, but since most people celebrate the festival at night (as it is also known as the moon festival), Wednesday is the public holiday for it.
I am not sure what there is going on in Stanley, but there is sure to be something as you are close to the beaches (that's where a lot of people go for the festival). The children light to play with paper lanterns (or plastic ones nowadays) and generally enjoy the moon, as it is supposed to be the biggest and roundest on the night of the festival. So enjoy!