Watch your back
In the original ten-month Roman calendar, days that marked the position of the moon were named: the calends was the first day of the month, or new moon, and the ides was the middle of the month, or full moon. 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 added days to each month throughout the year and replaced the extra month with an extra day—a bissextile day. But the ides continued to refer to the middle of each month, including March 15, the ides of March, when Caesar was assassinated.