Haridwar and Allahabad every 6 years. Ardha kumbha melas are not held at Ujjain and Nasik. 

Kumbhayoga (Astronomical Conjunction)  
The timings of the Kumbha melas are determined according to specific astronomical conjunctions. The planetary positions during which the melas are held at different locations are given below: 

Snanayoga 
During the mela in each of these places there are three very auspicious days for taking a bath in the sacred rivers. These auspicious days are connected with the phases of the moon. Generally the new moon and the full moon days are considered very auspicious. During these days the devotees take a holy bath in the sacred rivers. 

Scientific Nature of the Kumbha Mela 

Mela timings ensure better meditation  
The Kumbha mela generally recurs every 12 year in each of the mela sites. The mela cycle at these places synchronise with the different stages of the sun spot cycle which is known to have a very similar cyclic period of appx. 11.1 years. The sun spot cycle is known to enhance the electro-magnetic (EM) field of the Earth and its environment which in turn affects the bio-system. Of the many effects the EM field is reported to inhibit the regulatory systems like the nervous, endocrine, circulatory and respiratory giving rise to a condition quite similar to the inhibitory effects of meditation. In view of this naturally induced meditation physiology spiritual practices during the kumbhas is greatly advocated. 

Ritual Bathing  
The scriptures prescribe ritual bathing thrice a day in general, during the whole of the mela period and in particular on the new and full moon days. The enhanced EM fields during these periods magnetizes the river waters which thereby develops healing properties, hence the importance of bathing. 

What happens during the mela  
Taking advantage of the prevailing natural conditions that facilitates spiritual practices, people from all walks of life gather together at the mela either for the whole duration or part thereof. During these days everybody devotes themselves in intensifying their spiritual desires and practices. Prayer, worship, devotional singing, mantra chanting, meditation, study of scriptures, satsang, discourses from saints, self-less service goes on day and night. Devotees donate money freely as an act of charity and renunciation. The money is used to feed the thousands of monks, ascetics, sadhus, holy men who also attend these melas. Ritual bathing is done physically to take advantage of the healing property of the naturally magnetized