Does United require 24 hour check-in

United Airlines, like most major airlines, allows passengers to check in starting 24 hours before their flight’s scheduled departure time. However, 24-hour check-in is not a requirement but rather an option that provides convenience and helps ensure a smoother travel experience

Source: https://community.lumivero.com/s/question/0D5Jw00000dJ89wKAC/-18552000639-does-united-require-24-hour-checkin-unitedrequrment-policy