Passengers flying with Allegiant Air can begin the check-in process 24 hours before their scheduled departure. Allegiant Check In options include online, the Allegiant mobile app, or at the airport. For the latest details or any inquiries about check-in, you can reach Allegiant’s representatives at +1-702-505-8888 or +1-844-499-2038. https://www.flightslineup.com/blog/allegiant-check-in/