Hi Roberto, There are definitely times of the year like Oct/Nov/Mar and Apr that will give you a better chance at hooking up more fish as these are the generally accepted high season with conditions being near perfect, but it has become a common misconception that these are the only times to fish, GT fishing unfortunately does take a lot of hard work so if you willing to put the effort you will generally get results from Oct through to April. Booking wise obviously the "good" months get booked well in advance with April 2015 for us almost fully booked, so you need to decide when you have the time and put in your booking with the operator of your choice and hopefully the weather, GT gods and everything falls into place! I always maintain it is a Law of Averages, you have to have bad trips to have good trips, you have to catch small GTs to catch big GTs etc.
Hi Khairi, Obviously spring tides is generally the best period to fish for GTs as there is more water movement. I prefer new moon as opposed to full moon but this is a personal preference and it is a much debated topic what the best moon phase and tides are for GTs.
Cheers,
Nick