Anzac Day is commemorated on April 25th.
Will you be coming from the UK or are you planning to go from Fethiye?
There are many travelagencies in Istanbul (and all the backpacker hotels) who arrange it.
You have to book early though since they fill up quickly.
Bring many layers of clothes and a good warm sleeping bag since it get cold up there at night.
Some booze will do as well