Why do we use a 12 month calendar instead of following a 13 month lunar calendar?

The Gregorian calendar is the most widely used calendar in the world today. It is a reform of the Julian calendar, first proposed by the Calabrian doctor Aloysius Lilius, and decreed by Pope Gregory XIII, for whom it was named, on 24 February 1582 by papal bull Inter gravissimas.

Great arguments, but I'm not 100% convinced

That's a good question!

Sounds really interested

I've given you more infomation than you asked for. It includes why we use a time-base of 60, 24, 7 etc.

Please make sure you read it all.

"1 WHY DOES MONDAY COME ROUND SO QUICKLY?
How the numbers of the moon combined to make our week

...the seven-day week came about thanks to superstition, coincidence, human error, a need for order - and some elementary mathematics. These things aren't just responsible for the week having seven days, they also determined the orderof the days in the Western calender. ...

The moon calendar - and the number twelve
After the sun, the moon is by far the most predominant presence in the sky, and it goes through an obvious cycle... .
Archaeologists have found various clues that suggest that as far back as 30,000 BC the moon's cycle was being followed
closely. ...
There are good reasons why the moon would have been important to primative people. The period of full moon to full moon concides almost exactly with the periods between ovulations for women. ...
There is another reason why the moon would have had such an attraction as a timekeeper. The cycles of he moon are a natural way of dividing up the year. There are roughly twelve moons in a year, so that became the obvious dividing number... . ...
If the fluke of there being twelve moons per year first established this number as a fundamental measure of time, then the discoveries of the early civilisations in Egypt and Greece set the number twelve in stone.
Twelve is a conveniently small number, and it has other useful properties. One of the most important is that the number can be divided into two equal parts, or three, or four or six - which makes it a practical quantity for measuring and sndharing. ...

12: an abuntant number
All whole numbers have factors ... . The factors of twelve are 6, 4, 3, 2 and 1, which add up to 16. A number whose factors add up to more than the number itself is called 'abundant' and 12 is the smallest such number. ...mathematicians ... began to look at not just whether numbers are abundant or not, but how abundant they are. ... 12's abundancy is 16/12 or 1.33. It is beaten by 24 (36/24 = 1.5) amd this abundancy increases with every multiple of twelve up to 60, whose factors add up to 108. (108/60 scores a whopping 1.8) The number 60 is highly abundant because so many numbers divide into it, and this is why it became a popular base for counting. ...

The moon, the planets and the number seven
... From the earliest times, it would have been observed that ... stars were themselves slowly rotating, and that their rotation, like the sun's, took exactly a day. ... Some bright stars didn't follow the rest but moved at different speeds, and occasionally even looped back on themselves. ... They became known as the wanderers, for which the Greek word was planetes. Since these planets each had their own distinct movements, it was only natural too give them names. By the time of the Romans, they were known in today's English as the Moon, the Sun, Jupiter, Saturn, Venus, Mars and Mercury.
It was almost certainly because of this belief that there were seven so-called planets that this number aquireda mystical status. But, as with the number twelve, it took another coincidence to seal the significance of the number seven is linked to the cycle of the moon. From full moon to no moon is about fourteen days - two lots of seven. Full moon back to full moon is just over 29 days - not far from four lots of seven, or 28. And 28 is also a number loaded with mathematical significance,...

28: a perfect number
The factors of 28 are 1, 2, 4, 7 and 14 - ... these add up to 28. The Greeks ... labelled this number, and others with the same property, as 'perfect numbers'. ...
...
The belief that numbers with curious mathematical properties were in some way mystical would certainly have helped to establish those numbers as part of the culture. The perfect number 28 and the abundant number 12 were two of the main benerficiaries.
...
Linking the hours and the planets
We've seen how ... the numbers12 and 7 became crucial to the measurement of time, 12 because it divided up the year and, later, the day, and 7 because it divided up the month. The modern week was now ready to evolve, and the names of the days were to emerge because of obe further link between those two mystical numbers.
It had been established early in astronomical history that each of the planets took a different amount of time to complete a cycle ... . This led the planets to be given a hierarchy, with Saturn, the planet with the longest cycle, being the most senior.
...
The Egyptians were the first to divide the daylight into twelve hours, and then, aroung 1000BC, the Babylonians ... divided the day and night into 24 hours. Instead of naming the days after the planets, they decided to name the hours after the planets. The first hour was allocated to the most senior planet, Saturn, the second hour to the second planet, Jupiter, ..., repeating the cycle of seven planets throughout the 24 hours and continuing it into the following days.
...
Because 7 doesn't divide exactly into 24, the planet at the... [start] of each day changes. After seven days each of the seven planets has been at the... [start of each day] and ... the cycle begins again.
The planet at the... [start] of each day became known as its 'dominant' planet, and it became custom to name the day after thus dominant planet.
...
The Britons ... preserved the Sun-day [and the Saturn-day and Moon-day] ,but sacrificed the last four planet days to Anglo-Saxon Gods, Tiw, Woden, Thor and Frig. ..."

both, but must admit i would prefer the moon calender, i dont go with 13 being unlucky as all cultures have diff unlucky numbers, its all superstition

I guess it is both religious and cultural issue

maybe the 13th month would have been considered really unlucky so they just went with 12........i would have preferred about 26 months so we could give some more of them cool names like spoobembarch.....