There is a festival in Bihar named Chhat Puja in which people go to rivers, take bath and worship Lord Surya. They also worship a goddess called Chhati Maiya and that is why this festival name is Chhat Puja.
I want to know who was Chhati Maiya?
The origins of this festival are not clear but it seems to be related to the Surya Upasana that the eldest Pandav Yuddhishthir performed. The Section 3 of the Aranyaka Parva of the Mahabharat mentions the episode:
"Vaisampayana continued, 'Having heard from Dhaumya these words suitable to the occasion, Yudhishthira the just, with heart concentrated within itself and purifying it duly, became engaged in austere meditation, moved by the desire of supporting the Brahmanas. And worshipping the maker of day with offerings of flowers and other articles, the king performed his ablutions. And standing in the stream, he turned his face towards the god of day. And touching the water of the Ganges the virtuous Yudhishthira with senses under complete control and depending upon air alone for his sustenance, stood there with rapt soul engaged in pranayama. And having purified himself and restrained his speech, he began to sing the hymn of praise (to the sun).'
It is because of this ritual that Surya Dev apears and gifts him the copper vessel called Akshay Patra that provides unlimited food till Draupadi has eaten. The entire ceremony being performed while standing in water is quite suggestive of the Chhat ritual"
"Vaisampayana continued, 'Having obtained the boon, the virtuous son of Kunti, rising from the water, took hold of Dhaumya's feet and then embraced his brother's.
The terms Chhat & Chhati are colloquial terms derived from Shashti since this festival is celebrated on the 6th day of the Kartik month or the Kartik Shukla Shashti. Even though the festival is four days long but Chhat proper is on the 6th day which is also known as Surya Shashti.
This can be corroborated with the same term being used for the festival when it falls in the month of Chaitra and is known as Chaiti Chhati. This day forms a part of the Chaitra Navratri falling on the 6th day and is also known as Yamuna Chhath.
As regard Chhati Maiya, this is just a conjecture from my part but it could be either Katyayini the goddess worshiped on the 6th day of Navratri or the Vedic Goddess Usha who is closely associated with the Sun and has numerous hymns dedicated to her in the Vedas. Going by the association of the festival with Sun-worship the latter seems to be more probable.
There is also a tradition in Bihar which is called Chhati in which the birth of a child is celebrated after six days of birth.There are various folk-songs in Maithili, Angika, Bhojpuri and Maghadhi which clearly indicate that the Chhat is the celebration of the birth of Sun and most of women folk bear the great hardship of this Vrat for the well being of their children. And they wish their children should be like this Sun. This is the most popular festival celebrated with great sanctity by people and in this brahmin/priest or scripture have no role to play. It is played by a very ancient folk memory. The question arises from which period this memory comes in and why this memory is confined to the ancient Ang and Maghadh only.
The first question could be answered with the help of Rigveda in which great gods like Varuna, Yama and Indra were equated with Sun. Particularly Indra was worshiped as Sun (ref. RV.8.1(29). whose birth as Sun can be traced after his slaying of the most powerfull Vritra and establishing virtuous rule and rulership for society. Thus the festival of Diwali could be assumed as the victory of Indra over Vritra which is the day of birth of Sun (Indra) and the Chhat is the celebration of his birth exactly after six days. Later Hinduism forgot Indra and replaced his name with the avatara of Vishnu. But Diwali being a RigVedic tradition is celebrated by Buddhists, Jains and Sikhs as well.
The second question why only Ang and Maghadh retained those memory- because Indra belonged to Magh tribe of RigVeda and greatly helped by Ang. Many would raise question if Magh and Ang were really RigVedic tribes. I would raise and answer this question seperately soon