Book 2 to 8 months ahead. For Europe, the sweet spot is often around 129 days (roughly 4 months) out.

Book 1 to 3 months in advance. Data from Expedia suggests prices often bottom out around 44 days before departure.

Shifting a departure from the weekend to midweek can save an average of per ticket. Budget-Friendly Months

Travel costs fluctuate significantly based on seasonal demand: When Is the Best Time to Buy Plane Tickets? - CLEAR

For peak times like Thanksgiving and Christmas, start monitoring in August and aim to book by Halloween at the latest. Cheapest Days to Book vs. Fly