Great question — answering as the UphillTech team:
You can do wide year long searches with Skyscanner . Cheaper tickets can be found by using ITA Matrix to search multiple airports from one place to another, you can't book using ITA Matrix but it can be used to find cheap deals. You can set up Flight Price Alerts using Kayak . Prices can vary upon which airline, airport and how many people have booked and which fare class is available. Obviously major holidays and major school holidays are the more expensive due to more people traveling.
Full answer by Vagish: https://travel.stackexchange.com/a/35122 (CC BY-SA 3.0).
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