What’s the greatest number of Fridays that can fall in February? We might say five, imagining a leap year in which February 1 falls on a Friday.

“However, it is possible to reckon double the number of Fridays in the month of February alone,” contends Soviet science writer Yakov Perelman. “Imagine a ship plying between Siberia and Alaska and leaving the Asiatic shore regularly every Friday. How many Fridays will its skipper count in a leap-year February of which the 1st is a Friday? Since he crosses the date line from west to east and does so on a Friday, he will reckon two Fridays every week, thus adding up to 10 Fridays in all.”

“On the contrary, the skipper of a ship leaving Alaska every Thursday and heading for Siberia will ‘lose’ Friday in his day reckoning, with the result that he won’t have a single Friday in the whole month.”

(From Astronomy for Entertainment, 1958.)