Saw this article in the WP today. https://www.washingtonpost.com/travel/tips/google-flights-cheap-europe-holidays/ I wouldn't expect the daycount to be exact, but it does give an idea of when to look. YMMV
Aim to get way ahead of your European travel plans. Prices are lowest between 50 and 179 days before takeoff. The best of those deals tend to pop up 129 days out.
I was starting to feel like I needed to get on the ball booking my ticket for the spring, but at 258 days out, apparently not. Phew!
Thanks John, good to know.
Not sure that I get the best price because I haven't been that scientific (but good to know), my luck has been when watching the prices that I jump about 90 days out and I think it has worked out pretty well for me. Two months ago the flight I am taking in November were several hundred more than when I bought them about a week ago. (shortly after returning from the previous trip to Europe .... never too soon)
