For most major airlines in 2026, you can check in online starting 24 hours before your scheduled departure time. This standard window applies to carriers like Delta, United, American, and British Airways. However, there are notable exceptions depending on the airline and the route. For example, Southwest Airlines is famous for its 24-hour check-in rule because your boarding position is determined by how quickly you check in. Conversely, many low-cost European carriers like Ryanair and easyJet allow online check-in up to 30 days in advance if you have purchased a seat assignment, or 24 to 48 hours before if you have not. The online check-in window usually "closes" between 60 and 90 minutes before the flight departs to allow the airline to finalize the passenger manifest. Checking in as early as possible is highly recommended to secure your preferred seat, confirm your travel documents are in order, and—in the event of an overbooked flight—ensure you are higher up on the priority list to retain your seat during "bumping" situations.