Hi Kitty,
According to
http://www.lanzarotetourist.com/lanzarote-weather.cfm you should expect daytime temps in the region of 26 and night time temps of around 18, obviously there are no guarantees.
As for PDC being busy then I would say yes, expect bars to be busy in the evenings, the beaches to be full during the day and the strip to be heaving.
Hope you have a great stay.