Well the Sydney VIVID festival starts on the 25th of May and goes for 2 weeks, I went last year and enjoyed it a lot, so if your not sure what the VIVID festival is then details can be found at the link below. Basically its a festival of light and colour against the backdrop of the opera house, Circular Quay and the Rocks.
http://www.vividsydney.com/
http://www.vividsydney.com/