Found this article explaining why we have 24 hour days.

Other interesting date facts:

• The Babylonians counted days from sundown to sundown. The Jews who were dragged off during the captivity in the sixth century BC probably picked up the practice there, and have kept it ever since.
• The Babylonians may have also given the Jews their seven day week, as the pentatuch (the first five books of the bible) is widely agreed to have been composed during the captivity. The Babylonians (whose society was more than two thousand years old at the time Moses got fished out of the Nile) had a day for each astronomical body they knew of... the five planets, the sun, and the moon.
• September, October, November, and December got their names from the way they were written in the Julian (as in Julius Ceasar, who had it cooked up) Calendar: VIIber, VIIIber, IXber, Xber.
• The Julian calendar year started on March 25th.
• Slight mistakes in the Julian calendar built up over the centuries. By the sixteenth century it had got so bad Easter was being celebrated on the calendar nearly two full weeks before it was supposed to ocurr. The Gregorian calendar was created on comission of Pope Gregory XIII to correct these errors, and in 1582 ten days were simply excised from October to catch everyone up. You went to bed on October 4th and woke up on October 15th.
• Protestants, being highly suspicious of this new "papist" calendar, decided not to adopt it for nearly two centuries. When they did, in 1752, another day's worth of mistakes had accumulated. Eleven days were dropped from September of that year.
• Alaska didn't switch to the Gregorian calendar until the 1870s, when it was purchased from Russia by the United States.

See also this Ancestry.com article and this site on the 7-day week.