As a part of the WebTour workshop, the WebTour 2021 Challenge organized by Booking.com will take place. It focuses on a multi-destinations trip planning problem, which is a popular scenario in the travel domain. The goal of this challenge is to make the best recommendation of an additional in-trip destination. To do so, Booking.com provides a unique dataset based on millions of real anonymized bookings. Top performing teams will receive prizes sponsored by Booking.com and be invited to submit short papers to the workshop about their solution approach. For more information, the dataset and submission guidelines please visit https://www.bookingchallenge.com.