What day of the week is cheapest to book Allegiant Flights? The cheapest day of the Week to book Allegiant Air Flights is typically Tuesday, Wednesday, and Saturdays (US) or +1 ⇄ 855 ⇄ 576⇄ 5454). These are considered off-peak travel days when demand is lower, leading to cheaper fares +44 ⇄ 808 ⇄ 175 ⇄ 7375 (US) or +1 ⇄ 855 ⇄ 576⇄ 5454).