Why are the ‘cheapest days’ cheap? It’s a matter of demand; most people don’t want to fly on a Tuesday, Wednesday or Saturday so airlines have trouble filling their planes. And because empty seats bring in no revenue, airlines fill them by lowering prices. Cheapest days to fly, domestic: Usually Tuesdays, Wednesdays, Saturdays Cheapest days to fly, international: Weekdays are usually cheaper than weekends. When it comes to holidays, the general rule is the closer to the holiday you fly, the cheaper the ticket. During Christmas, the cheapest days to travel are the 23rd and 25th of December, with the 31st of December being the cheapest day to return. On domestic routes, fares for overnight red-eye flights are often lower than for daytime flights—but you have to put up with the miseries of sitting up all night in a tiny economy seat.