TeachMe You ask two further questions: Where can you find out about high season and when is the 'sweet spot' to book.
In my opinion the answers are: Nowhere and Never.
Firstly because High Season now means anything the carrier chooses to call High Season. Whilst school holidays and the latitude of the sun remain the key arbiters, due to the factors mentioned in the thread, the season can move.
Secondly, due to the carrier's computers reviewing prices ever day - they have eliminated the sweet spot. There used to be one, when the carrier manually checked the sold/unsold and the time to departure, made a mgmt decision and telexed changes to their sales offices and agents. But that was a long time ago!!