If you’re one of them, check this out: According to someone on the MIMlist listserv who used to work for an airline, Sundays tend to be your best bet for oversold flights, especially on major airlines flying from hub to hub. Monday mornings and Friday evenings also tend to sell out. Because it depends on the frequency of flights an airline has going to a particular city, the most likely to oversell are airlines with the fewest number of flights per day to that city. Also, because some planes have to reduce their loads on hot days, some may have to have empty seats even if the flight isn’t oversold when the mercury soars.
How it works--Do a search for a flight, and here's how you interpret the results:
F9 J9 C9 -- first / business is pretty open
F0 J0 C0 -- first /business sold out
The number will fluctuate between the above
depending on the bookings in that class; you might see J7 for example. F is first, J is business, C is discounted business.
Then the next set of details for each flight features the the booking levels, by fare class, in economy. For example, if you look at Toronto-LaGuardia, July 1, flight 4899, you'll see:
Y7 B7 H7 K7 M3 L2 V0 W0 Q0 G0 O0 S0 N0
(There is no F, J, C since this is an all-economy flight)
Interpreting the above:
Y is full fare economy
B, H , K, L etc are discounted economy seats. Thereis very little L class availability (meaning the really cheap seats are gone), and even full-fare economy (Y) is selling, since it isn't 9 (9 is the max you'll ever see, even if there are hundreds of seats in that category)
If you see a lot of 0s, chances are the flight is overselling.
He adds that you can also use the same tool to study your chance for an upgrade, if you are trying to use upgrade coupons, by looking for flights that have lots of C class availability, since that is the class usually assigned for seat upgrades.
Whether you're looking to get bumped, or, like me, are looking for an empty flight where you can stretch out a bit, this could come in handy.