on what の意味は？

下記の文章を訳していただけませんか？

Because of the long-haul flight, most visitors stay for longer (average 20 days) and want to see as much of the country as possible on what is often seen as a once-in-a-lifetime visit.

特にon whatの部分が訳しづらいので２文に分けて解説していただけると嬉しいです。