How do you know when an airline is going to have a Sale? Actually, much of the time it is quite easy because airlines hold special promotions at fairly predictable times (Black Friday, after Christmas and so on). However, they do have quite frequent bursts of activity throughout the year and these can take you by surprise.
Sales are actually quite difficult for an airline to handle because they are offering a discount on something with no fixed price. Fares increase as seats are sold so a fare offered at a 20% discount during a Sale could still cost more than the same fare a week ago when fewer seats had been sold. That would not look good.
One technique used by some airlines and hotel chains is to gently edge up the base-price in the week before a special promotion. The price will still increase as sales are made but the starting price has been increased. Then, on the day the “Amazing Sale” is announced, the base-price is reduced again. Some of the fares will be genuine bargains but not all will have quite the amazing reductions claimed.
If you are looking to buy flights for next summer and have been checking up on fares periodically to see when they might reduce, you might notice a sudden slight increase in the week before Christmas. Unless there is just one flight to the destination you are looking at, there is no need to panic. Not many people buy tickets in the week before Christmas so airlines and hotel chains can do their own seasonal decorations on prices – before they suddenly reduce them on Boxing Day.