Why do we have months?
(Lansing State Journal, November 4, 1992)
The months were originally introduced to follow the cycles of the Moon, the second most visible body in the sky. In contrast, both days and years are based on the apparent movement of the Earth around the Sun. For the ancients, an accurate measurement of the cycle of the Moon was much simpler to make than that of the solar year. So most of the ancient civilizations used a lunar calendar. However, the lunar month does not correspond to an exact number of days; it varies continuously and on average lasts about 29 days 12h 44 min 2.8s. Rapidly, the ancients adopted alternating months of 29 and 30 days, with an extra day every 30 months to account for the 44 min. The year was then 12 months or 354 days.
But, because this year was 11.25 days short of the solar year, the seasons would shift from one year to the next, making it difficult to plan for sowings and harvests. The Egyptians were the first to abandon the lunar calendar and to introduce twelve months of 30 days plus five days at the end of the year, to keep the seasons recurring at the same year after year. Later, the Romans distributed the five days over the year with 12 months of alternatively 31 and 30 days for leap years and February with 29 days otherwise. However, when the eighth month was dedicated to the emperor August, a 31st day was added to make it the equal of July, dedicated to Julius Caesar, and removed from February which became the short month with only 28 or 29 days that we know today. Although the solar calendar is used all over the world for civil business, the lunar calendar is still followed in many religions including Islam, Judaism, and Catholicism (for fixing the Easter date).