It is a major teerth and festival. It is celebrated in a cycle of approximately 12 years, to celebrate every revolution Brihaspati (Jupiter) completes, at four river-bank sites: Prayagraj (Ganges-Yamuna-Sarasvati rivers confluence), Haridwar (Ganges), Nashik (Godavari), and Ujjain (Shipra). The festival is marked by a ritual dip in the waters. It is also a celebration of community commerce with numerous fairs, education, religious discourses by saints, mass gatherings of monks, and entertainment.

This mela along with other large gatherings enables Sanaatan Society to re-orient itself to account for changing times. Many saints communicate with people is an easy-to-understand language and re-enforces the Sanaatan Philosophy. Some of the significant observations about Kumbh Mela:

  • This eliminates the need for reading and writing skills.
  • Many debates also take place, in which changes to customs and traditions may be discussed as needed and corrective actions/ recommendation made for the society to follow.
  • Guru-Shishya relationship is also established.
  • People from all areas come and inter-mingle, which helps in creation of an accepting/ tolerant society.
  • People of all castes/ varns join, which nullifies the theory about hierarchy of castes in Sanaatan Society.
  • Lot of trade happens, which enables adoption of good technical know-how and learning.

Though not sure when it started, as per some account, Shri Adi Shankaracharya started it. [By some authentic source, Adi Shankaracharya was around 500 BC and not 500 AD].

Some links:

https://en.wikipedia.org/wiki/Kumbh_Mela Wikipedia Link
https://isha.sadhguru.org/us/en/wisdom/article/kumbha-mela-the-greatest-gathering Sad Guru explains science behind this
https://www.kumbhmela.com/history-of-the-kumbh-mela/# Has references to scriptures.