Airlines give better fares to people who advance book because they are trying to encourage people to book as early as possible. If the airline were to lower fares just before flight time there would be a flood of people (on random flights) at the last minute. Airlines need an accurate estimate of the number of people and amount of baggage on a flight so that they can load the proper amount of fuel. (Meals and beverages also have to be loaded.)
Moreover, people who book at the last minute are usually flying on business, and therefore the business is paying for it. People flying for pleasure usually know weeks or months in advance, and can't afford the prices that a business would pay. Thus it is to an airline's advantage to set rates according to the major differences between business and leisure travelers:
- Business travelers fly mostly between 9 and 5, whereas leisure travelers can fly offpeak hours.
- Business travelers buy tickets on very short notice, whereas leisure travelers plan trips well in advance.
- Business travelers do not stay over a weekend (= Saturday night), whereas leisure travelers do.
So airlines typically give discounts for people who stay over a weekend, flying offpeak hours, and purchasing tickets 7 days, 14 days, 21 days or 30 days in advance. Such fares are known as excursion, discount, or supersaver fares.
For example, US domestic excursion fares require that you buy your roundtrip ticket 7 days, 14 days, or 30 days in advance, and that you stay over a weekend (usually Saturday night, though sometimes Sunday night as well). Some may also restrict the travel to a 30-day maximum stay. Stopovers aren't allowed, except for the purpose of connection (if you want a stopover, you'll have to pay extra). Some fares may be limited to a particular routing (e.g., routes with connections cost less than nonstop flights). You may also be limited to flying during offpeak hours, with flights during the busiest times of the day costing more. Tickets are usually non-refundable and non-transferable, and may either be non-changeable or have a $35 change fee. The price is usually the average of the two one-way tickets. (E.g., a 2-week advance PGH/BOS advance ticket is around $200 this way.) Tickets must be purchased within 24 hours of the reservation.
For international excursion fares, also known as apex fares, you must book at least 21 days in advance of the flight, and you may have to purchase the tickets at the time of the reservation. Minimum stay is usually 7-10 days, and maximum stay can range from 3 months to a year. Midweek travel (Monday, Tuesday, Wednesday, or Thursday) is usually cheaper. Tickets are non-refundable and non-transferable. Fares often depend on the seasons.