You will likely get assigned a seat during check-in (though you should still skip it if the airline tries to tack on a fee) or, at the very least, at the gate. Again: You don't have to select a seat in order to get a seat. You just might get stuck with a suboptimal one if you choose to skip it.

Whether you pick your seat during check-in depends on the airline's policy and the type of fare you purchased. Most major "legacy" carriers allow you to select your seat for free starting 24 hours before departure during the online check-in process, provided you haven't already paid for a specific seat earlier. However, on "Basic Economy" or "Ultra-Low-Cost" fares (like those on Spirit, Frontier, or Ryanair), seats are often randomly assigned by the computer at check-in; if you want to pick a specific spot at that stage, you will typically be charged an additional fee. Some airlines also hold back a small percentage of seats (such as those in exit rows or with extra legroom) that only become available for purchase or selection during the check-in window. If the flight is nearly full and you have not selected a seat in advance, the system may automatically assign you whatever is left. For the best selection, it is recommended to check in as close to the 24-hour mark as possible. If you are unable to select a seat online, an agent at the airport counter can often assist you, though their options may be limited by the remaining availability.