In the context of the travel industry, "checking in" usually refers to two different things: airlines and hotels. For airlines, you can almost always check in exactly 24 hours before your flight. This is a standard window during which you can confirm your attendance, select or change your seat, and receive your digital boarding pass. Checking in the moment this window opens is highly recommended, especially for airlines like Southwest that assign boarding positions based on check-in time. For hotels, the phrase "24-hour check-in" is less common. Standard hotel check-in is usually between 3:00 PM and 4:00 PM. While some modern or airport-centric hotels offer "24-hour check-in" (meaning their front desk is staffed all night), you generally cannot check into your room at 8:00 AM without paying for an "early check-in" fee or the previous night's stay. However, if you arrive late at night, a 24-hour front desk ensures you can get your key regardless of the hour. Always notify your hotel if you plan to arrive after midnight to ensure they don't mark you as a "no-show."