We have 30 days in a month because it takes almost 30 days for the moon to complete it's phase. We have 365 days for a year because, earth takes almost 365 days to revolve round the sun. But! Have you ever wondered why we have 7 days in a week! Here is the answer...
Though many civilizations adopted a 7 day week... Babylonians were probably the first ones to popularize a 7 day week. Our ancestors felt that a change in the moon can be visibly noted for every 7 days. On the first day we can watch a small cresent in the sky. On the seventh day, a waxing half moon can be found. On the 14th day, it would almost be a full moon. By the 21st day the moon would be waning into half moon and by 28th moon we can see just a cresent which would soon turn into a new moon.
The moon takes 29.5 days to complete the phase from moon to new moon. But the Babylonians would have only 28 days combined in four weeks. So to catch up the moon again, they used to add one or two days more to the final week. Having said enough about the reason behind a seven day week, which day should be considered as the first day of the week would be an interesting question. Some people start counting the week with Sunday while some others consider Monday as the first day. To avoid any further confusion the 'International Organization for Standardization (ISO)'has declared MONDAY as the first day of the week.
Origin of names for a week:
MONDAY: Monday comes from the noun 'moon'.
TUESDAY: Tuesday is named after a scandinavian god named Tyr. Tyr is the god of war and equallent to the Roman god Mars! So this day of the week can be said to be dedicated for the planet Mars.
WEDNESDAY: Wednesday comes from the word 'Woden's day'. Woden was a chief god and a leader of hunting. He is thought to be equal for the Roman god - Mercury.
THURSDAY: Thursday derives it's name from the Norse god 'Thor'. Thor is the god of thunder and lightening. Romans has dedicated this day to Jupiter who is also considered to be the god for thunder.
FRIDAY: Friday is named after an old English goddess called 'Frigg'. She is often considered to be equallent to Roman goddess for beauty - Venus.
SATURDAY & SUNDAY: As the names give us the hint... Saturday and Sunday were named after Saturn and Sun!