The day starts in Kiribati. If you start there, on the most eastern island, west of the dateline, and you move to the most western island east of the dateline, which is Attu Island, and you cross the dateline at midnight, you can experience a 51-hour day.
Start at 00:00 in Kiribati, the most eastern island of Kiribati, on April 1. I think for that distance, you would need a plane. 24 hours later, you cross the dateline, and the date remains April 1. It's 00:00 hours, midnight. 24 hours later, at 21:00 hours, April 1, you arrive on Attu Island, where the day will end three hours later.