You'll only know the optimal time to book after the fact. There are just too many factors at play.
Generally, for intercontinental flights, it is best to book at least 3 months prior to departure. Most airlines will allow you to book up to 11 or 12 months in advance. This means you can book as early as right now or as late as end of May.
In general it is best to start by gauging typical prices for the route. This can be accomplished by doing repeated search for trips on various dates (including those that wont work for you) to get a feel for what the cheapest possible fare is going to be.
Once you've got a feeling for the price range you should start to monitor the dates you actually can use. If you find a price near the bottom end of the range, book it.
If you have the option of not traveling, you can wait and hope for a good last minute fare or otherwise hold out for 'just the right price'. However, if you have to travel and especially if you are traveling on a holiday, do not wait too long.