In the states Southwest Airlines doesn’t assign seats at all, you’re assigned a place in the boarding line and pick any seat. There are upgrades to get in sooner, but for normal folks the order you check in is your place in line, so it is worth checking in 24 hours early (the earliest possible).
Some airlines don’t have seat assignment until you print your boarding pass
In the states Southwest Airlines doesn’t assign seats at all, you’re assigned a place in the boarding line and pick any seat. There are upgrades to get in sooner, but for normal folks the order you check in is your place in line, so it is worth checking in 24 hours early (the earliest possible).