What is the reason behind considering number 12 as special?

Question

Purna Kumbh Mela is celebrated every 12 years. It is also believed that spiritual progress happens in practitioners in the cycle of 12 years i.e., it takes 12 years of disciplined practice to change a habit. Mantras, Japas and Pranayamas are practiced in multiples of 12.

Why we use number 12 and what are the secrets of number 12?

EDIT:

More examples of 12:

• A man controlling the seminal fluid for twelve years develops a special power. He grows a new inner nerve called the nerve of memory. Through that nerve he remembers all, he understands all. (source)
• Where was Arjuna for 12 years when he was exiled?

Answer 1

It takes 12 years for Guru (Bruhaspati, Guru of Devas) aka Jupiter to complete its 1 year (one revolution around sun).

According to Srimad Bhagvatam 5.22.15:

Situated 1,600,000 miles above Mars, or 10,400,000 miles above earth, is the planet Jupiter, which travels through one sign of the zodiac within the period of a Parivatsara. If its movement is not curved, the planet Jupiter is very favorable to the brāhmaṇas of the universe.

Here Parivatsara is one of the name of a year. It's stated Jupiter travels one zodiac sign in a year. And as we know, there are toal 12 zodiac signs. So, it takes 12 years for a Jupiter to complete 1 revolution around sun.

Just as humans celebrate birthday every 1 human-year, so we celebrate divine festivals like kumbh-mela every 1 Bruhaspati-year.

Also, see 12's significance in comment posted by @Ajay above which links to yahoo answers.

From Mahabharata Vana Parva chapter 134, Ashtavakra says importance of each number. In regards to twelve, he says

Ashtavakra said, Twelve months compose the year; twelve letters go to the composition of a foot of the metre called Jagati; twelve are the minor sacrifices; and twelve, according to the learned, is the number of the Adityas.'

Above link has more details on importance of number twelve.

Answer 2

The secret of 12 is the 12 Adityas, of whom Vishnu/Vamana is the eldest and Indra is their king. These are 12 forms of Sun/Surya Narayana. The 12 Adityas are basically the monthly suns which is the ancient word for the earth moon barycenter for lunar month. These are also called as the 12 purushas pertaining to the 12 lunar months of the year.

These 12 also rule the 12 houses of horoscope of every individual and in reality the manifestation of 7 planets(days of the week) or 7 chakras or 7 sages/Saptrshis. 1 Sun/Purusha/Soul/Father + 1 Moon/Prakriti/Mind/Mother + 2x5 planets/senses/elements.