our calendar

We follow the Gregorian calendar. It turns out that it wasn't some guy name Gregory who came up with the scheme. Rather, Pope Gregory XIII declared the scheme proposed by Aloysius Lilius as the official calendar for the Church. The Church needed some consistent way to set a date for Easter. Even with our calendar, Easter is no easy thing like a specific date or day every year. Instead, Easter is set as the first Sunday after the first full moon of Spring.

Our calendar year is the average time between "vernal equinoxes" -- the moment when the sun is at earth equator in its passing from south to north in March. At this time, the day and night are approximately of equal length. The calendar is an imperfect predictor because the earth's orbit is not mechanically exact. What our calendar gives us is an approximation and every so many years, we adjust it with an extra day in February--the extra day being February 24 with the month having 29 days.

You can read all about it and have your own headache at Webexhibits http://webexhibits.org/calendars/year-astronomy.html