For over 4,000 years we have relied on the duodecimal mathematics of the Bronze Age Babylonians, who built the first great cities in what is now Iraq. They saw the number 12 as much more important than 10 because it is divisible by 2, 3, 4 and 6, making it particularly versatile in mathematical calculations.

What’s more, the lunisolar calendar, based on observations of both the Sun and the Moon, used 12 lunar phases per year (with a 13th ‘leap’ month added every two or three years).

So 12 was the numerical cornerstone of the Universe. Early civilisations, including Egypt, divided both sunlit day and night into 12 parts – in total, 24 hours in a day.

Source:

http://www.historyextra.com/qa/why-are-there-24-hours-day