"Dayzed" and Confused?!

Why do some months have 30 days and others have 31 days?

This is waaaaay more complicated than I ever thought it would be so here's what the resources I accessed have to say directly. The bottom line is that it has to do with the moon, an emperor, and a Pope:

The calendar is based on three key astronomical events.

• A day, which is the time from one sunrise to the next sunrise — one complete rotation of the Earth.
• A year, which is approximately 365.24 days — one complete orbit of Earth around the Sun.
• A month, which is approximately 29.53 days — one complete orbit of the Moon around the Earth.

Since these time spans are not easily divided, calendars have always been imperfect. Some were rooted in tradition, while others evolved as humankind gained a greater understanding of science and astronomy. Some calendars, like the Christian calendar (which is the primary calendar in use today) focused on the Earth’s orbit. Others, like the Islamic calendar focused on the Moon’s orbit. Still others, like the Jewish calendar and Chinese calendar, combine both.

Most calendars are based on astronomical events. From our perspective on Earth, the two most important astronomical objects are the Sun and the Moon, which is why their cycles are very important in the construction and understanding of calendars.

Our concept of a year is based on the earth’s motion around the sun. The time from one fixed point, such as a solstice or equinox, to the next is called a tropical year. Its length is currently 365.242190 days, but it varies. Around 1900 its length was 365.242196 days, and around 2100 it will be 365.242184 days. f

Our concept of a month is based on the moon’s motion around the earth, although this connection has been broken in the calendar commonly used now. The time from one new moon to the next is called a synodic month, and its length is currently 29.5305889 days, but it varies. Around 1900 its length was 29.5305886 days, and around 2100 it will be 29.5305891 days.

Note that these numbers are averages. The actual length of a particular year may vary by several minutes due to the influence of the gravitational force from other planets. Similarly, the time between two new moons may vary by several hours due to a number of factors, including changes in the gravitational force from the sun, and the moon’s orbital inclinatio

The motion of the Moon in its orbit is very complicated and its period is not constant. The date and time of this actual observation depends on the exact geographical longitude as well as latitude, atmospheric conditions, the visual acuity of the observers, etc. Therefore the beginning and lengths of months in these calendars can not be accurately predicted.

The problems of creating reliable lunar calendars may explain why solar calendars, having months which no longer relate to the phase of the moon, and being based only on the motion of the sun against the sky, have generally replaced lunar calendars for civil use in most societies.

The Gregorian calendar, like the Julian calendar before it, has twelve months:

1. January, 31 days
2. February, 28 days, 29 in leap years
3. March, 31 days
4. April, 30 days
5. May, 31 days
6. June, 30 days
7. July, 31 days
8. August, 31 days
9. September, 30 days
10. October, 31 days
11. November, 30 days
12. December, 31 days

The average month in the Gregorian calendar has a length of 30.4167 days or 4.345 weeks in a non-leap year and 30.5 days or 4.357 weeks in a leap year, or 30.436875 days in a mean Gregorian month overall (365.2425 ÷ 12).

SOURCE:

Calendars Through the Ages http:www.webexhibits.org/calendars

Gregorian Calendar http://en.wikipedia.org/wiki/Gregorian_calendar