my word of the day from dictionary dot com
\bahy-SEKS-tuh s, bi-\
1. February 29th: the extra day added to the Julian calendar every fourth year (except those evenly divisible by 400) to compensate for the approximately six hours a year by which the common year of 365 days falls short of the solar year.
This odd day was inserted after the sixth day before the kalends of March, i.e., after the 24th of February, and was not counted as an addition to the year, but as a sort of appendix. Hence the sixth of the kalends of March was called bissextus, or double sixth, which root is still retained in our word bissextile, though the day is now added at the end of February.
— E. S. Burns, “History of Chronology,” The Popular Science Monthly, April, 1881
Bissextus comes from the Latin term bissextus diēs meaning “intercalary day.” It was so called because the 6th day before the Calends of March (February 24th) appeared twice every leap year.