SERVILIA: The Jewish holiday of Hanukkah occurs around the time of Christmas, but the exact date varies from year to year.

RALPH: That’s because Hanukkah is scheduled according to the ancient Jewish lunar calendar, which advances 10 days each year.

SERVILIA: And just like the ancient Romans, the modern Jewish calendar  periodically adds an extra month to the year in order to keep their lunar  calendar roughly in sync with the solar year.

RALPH: For example, if Hanukkah starts on December 24th this year, it will start on December 14th next year, and December 4th the year after. Then an extra month is added so that the following year Hanukkah will once again start around the 24th.

SERVILIA: Islam also uses a lunar calendar. Their ninth lunar month, Ramadan, is observed by fasting from sunrise to sunset for the entire month.

RALPH: However, the Islamic religious leaders never add an extra month to their year. This means that Ramadan advances 10 days every year, rotating through the entire year over the period of 35 years.

SERVILIA: This can make fasting especially challenging for Muslims living at higher latitudes, such as London. When Ramadan falls in mid-winter, they only have to fast for the short 8 hours of daylight; but when it falls in midsummer, they have to fast for 16 hours each day.