Johns Hopkins Magazine
Johns Hopkins Magazine Current Issue Past Issues Search Get In Touch
G O L O M B' S    G A M B I T S &trade
Calendar Oddities
By Solomon Golomb '51

A. The Gregorian Calendar
Throughout the history of the American republic, the official civil calendar has been the Gregorian calendar.
1. For whom is the Gregorian calendar named?
2. In what year was it first introduced?
3. What was the name of the calendar it replaced?
4. An "ordinary year" has 365 days; a "leap year" has 366 days. What is the rule, in the Gregorian calendar, for which years are leap years?

B. Friday the 13th (for triskaidekaphobes)
1. What is the largest number of Fridays the 13th that can occur in a single calendar year? How does this happen?
2. What is the shortest time interval in which there can be two Fridays the 13th? How does this happen?
3. What is the shortest time interval in which there can be three Fridays the 13th? How does this happen?
4. What is the longest time interval that can elapse without a Friday the 13th? Can there be a calendar year with no Friday the 13th?

C. Miscellany
1. In the United States, 10/6 means "October 6," while in Europe it means "June 10." In one calendar year, how many days have ambiguous dates, if you don't know which system is being used?
2. Can you give a more sensible interpretation of the relationship OCT. 31 = DEC. 25 than "Halloween = Christmas"?
3. What do the holidays New Year's Day, Groundhog Day, Cinco de Mayo, D-Day, and Armistice Day have in common that is not shared by Valentine's Day, Flag Day, or Independence Day?
4. Our history books say that George Washington was born on February 22, 1732. On the day he was born, what date appeared on the local Virginia newspaper?

D. Subtleties
1. From the rules for the Gregorian calendar, prove that the probability that Christmas falls on Wednesday (over the long term) is not equal to one-seventh.
2. On what day of the week is the 13th of a month most likely to occur? (The answer may surprise you.)
3. William Shakespeare died in England on April 23, 1616. Miguel de Cervantes (author of Don Quixote de la Mancha) died in Spain on April 23, 1616. Yet they died more than a week apart. How was this possible?
4. In the Soviet Union, the October Revolution was commemorated annually in November. Why?

Follow this link to solutions.

Return to September 2007 Table of Contents

  The Johns Hopkins Magazine | 901 S. Bond St. | Suite 540 | Baltimore, MD 21231
Phone 443-287-9900 | Fax 443-287-9898 | E-mail