The festival is traditionally credited to the 8th-century Hindu philosopher and saint Adi Shankara, as a part of his efforts to start major Hindu gatherings for philosophical discussions and debates along with Hindu monasteries across the Indian subcontinent. However, there is no historical literary evidence of these mass pilgrimages called "Kumbha Mela" prior to the 19th century. There is ample evidence in historical manuscripts and inscriptions of an annual Magha Mela in Hinduism – with periodic larger gatherings after 6 or 12 years – where pilgrims gathered in massive numbers and where one of the rituals included a sacred dip in a river or holy tank. The weeks over which the festival is observed cycle at each site approximately once every 12 years based on the Hindu luni-solar calendar and the relative astrological positions of Jupiter, the sun and the moon. The gap between Prayag and Haridwar festivals is about 6 years, and both feature a Maha (major) and Ardha (half) Kumbh Melas. The exact years – particularly for the Kumbh Melas at Ujjain and Nashik – have been a subject of dispute in the 20th century. The Nashik and Ujjain festivals have been celebrated in the same year or one year apart, typically about 3 years after the Haridwar Kumbh Mela. The Kumbh Melas have three dates around which the significant majority of pilgrims participate, while the festival itself lasts between one to three months around these dates. Each festival attracts millions, with the largest gathering at the Prayag Maha Kumbh Mela and the second largest at Haridwar. The festival is observed over many days, with the day of Amavasya attracting the largest number on a single day. The Kumbh Mela authorities said that the largest one-day attendance at the Kumbh Mela was 30 million on 10 February 2013 and 50 million on 4 February 2019.