India's largest airline, IndiGo, on Tuesday announced a special offer to mark its 20th anniversary, giving customers a chance to win one of 20,000 free domestic flight tickets.As part of its 'Happy IndiGo Day' campaign, called IndiGoFree, the airline will refund the fare paid by selected customers, up to Rs 10,000 per PNR, for eligible domestic bookings made between August 4 and August 6, 2026.The offer is available only on bookings made through IndiGo's direct channels, including its website, mobile app and AI virtual assistant, 6Eskai.The airline, which is celebrating two decades of operations, currently operates a fleet of more than 430 aircraft, flies nearly 2,200 daily services, and connects more than 95 domestic and 40 international destinations.IndiGo said it served over 123 million passengers in FY26.How the offer worksCustomers who book eligible domestic flights during the offer period will be entered into a random selection process. Those selected will receive an email and WhatsApp message inviting them to answer a simple question.Each selected customer will get three chances to answer correctly. Those who do so will receive a refund of their booking fare, capped at Rs 10,000 per PNR, within three to five business days.The offer is valid for both Economy Class and IndiGoStretch bookings, including one-way and return journeys on domestic routes.IndiGo said the offer is subject to terms and conditions available on its website.