What does bissextile mean?

Bissextile has nothing to do with issues of gender, sexual stimulation or obsessing over the number 6. It simply refers to a leap year – a year containing February 29th and therefore a total of 366 days. A bissext (or bissextus) refers to the extra day itself, but leap day is more widely used. February 29th can also be technically referred to as an intercalary day. Intercalary is an adjective of Latin origin, which describes the insertion of an extra day in the calendar in order to harmonize it with a solar year.

The word bissextile in fact has its origins in the 16th century, deriving from the Latin term “bissextilis”, meaning 'having an intercalary day'. In the early Roman calendar an extra month was added every few years to ensure that agricultural festivals and holidays remained at the appropriate time of year. Julius Caesar created the Julian calendar which corrected this 'drift' by adding days to each month throughout the year, and the intercalary day in leap years.

This bissext was the 6th day before the 'Calends of March' (i.e. 24th February, counting back from the beginning of March) and which occurred twice in a leap year. Hence the term is based on Latin “sextus” – 'six' – and “bis” – 'twice' – and means 'twice the 6th day'. The word bissextile is relatively rare in English usage, but emerges from obscurity every 4 years. The phrase “année bissextile” is, however, a commonly used French term for a leap year. The next “année bissextile” is next year, 2020.