Ambubachi is celebrated every year at Kamakhya Temple situated at the Nilachal hill in Guwahati, the capital city of the state of Assam in India. The fair falls in the Assamese month of Aahaar corresponding to Sanskrit month of 'Aashara'. This is generally the month of June as per Gregorian calender and is one of the monsoon months for Assam.
The word Ambubachi, pronounced as Ambubasi in assamese is derived from The Sanskrit term Ambuvācī. Ambu literally means water or four and therefore ambubachi literally mean “expression/commencing of water,” referring to the commencement of monsoon and refurbishment of Earth’s ground waters. It may also directly refer to the four day in the month of 'Aashara' starting from 10th days in Aashara month when the earth is supposed to be unclean and agriculture is prohibited Many believe that this is marking of Kamakhya’s annual cycle. This comes from the mythological stories of Sati, whose Yoni(genitals/womb/female organ of reproduction/source) fell here and therefore revered by devotees as very sacred place and a symbol of creation.
Many of tantric fraternity believe that mother earth goes through menstrual cycle during this period which starts from 4th part/phase of Mrigashira Nakshatra(Orion constellation) to 1st phase of Adya Nakshatra and therefore finds this time suitable for tantric rituals.
Kamakhya's Ambubachi mela is the aggregation of the beliefs and rituals surrounding this cycle and period of 'impurity' (or fertility?) and celebrated by huge congregation of devotees from Assam, West Bengal, North Indian states as well as from foreign countries.
In recent years administration has done a commendable job in arranging shelters for devotees, arranging buses for commuting between temple and these shelters, free foods & water to these devotees and along with these also been able to maintain the law and order in a time when lakhs of people visit the temple.