This Wikipedia page mentions that Draupadi and Kunti performed the Chhath Puja. However, the same page also mentions it to be a folk festival, which is celebrated on the Shukla Shashti of Kartika Masa (i.e. 6 days after Diwali). I want to know if there are any mentions of this festival in Mahabharata and our Puranas or if is it just a folk festival.
14th century Maithilī paṇḍita Rudradhara mentions the Vivasvat-ṣaṣṭhī vrata (popularly known as Chat-Pūjā) in his Smṛti compendium called Varṣakṛtya.
He states that in this vrata, the votary is to observe restraint on Kārtika śuklā pañcamī, observe fast on the next day accompanied by offering arghya to Sūrya during sunset in the banks of a waterbody after worshipping Him with fruits, flowers, scents, lamps, incense & foodstuff accompanied by music & dance, keep awake that night through music & dance, then on the next day, again offer arghya to Sūrya during sunrise & observe His special worship & fast thoughtout the day, & finally terminate the fast on Kārtika śuklā aṣṭamī accompanied by feeding of brāhmaṇas & hearing of the vratakathā.
Chat-Pūjā is also mentioned as Pratihāra ṣaṣṭhī by 15th century Maithilī paṇḍita Vidyāpati in his Smṛti compendium (also named) Varṣakṛtya. However, it wasn't possible to procure a copy of it.
The vrata-katha mentioned by Rudradhara (purportedly from the Skandapurāṇa) is extremely simple at the best. A person is born wretched & afflicted with leprosy, so he approaches a brāhmaṇa for remedy. The brāhmaṇa advises performance of Vivasvat-ṣaṣṭhī vrata, the person observes it & gets rid of his leprosy & misfortune. Another poor brāhmaṇa hears about this vrata & becomes rich after observing it. And thus the vrata gets popular among the masses.
Chhath Puja or Surya Shashthi Vrata is not a folk festival. It is mentioned in the Samva Purana, which is a Saura Upapurana. This is an important Upapurana, as it is included in all lists of 18 Upapuranas (For example 18 Upapuranas mentioned in Srimad Devi Bhagavatam 1.3.12-17). It describes many rituals regarding the worshipping of the Sun God.
उपवासेन षष्ठ्यां च सर्वपापौः प्रमुच्यते।१७
एकहारपरो भूत्वा षष्ठ्यां योऽर्चयते रविम्॥२०
नियमव्रतधारी च रवेर्भक्तसमन्वितः।
सप्तम्यां च महाप्राज्ञः सोऽश्वमेधफलं भवेत्॥२१॥
अहोरात्रोपवासेन पूजयेद्यस्तु भास्करम्।
सप्तम्यामथ षष्ठ्यां च सूर्यलोकं स गच्छति॥२२॥
upavāsena ṣaṣṭhyāṃ ca sarvapāpauḥ pramucyate। 17
ekahāraparo bhūtvā ṣaṣṭhyāṃ yo'rcayate ravim॥20
niyamavratadhārī ca raverbhaktasamanvitaḥ।
saptamyāṃ ca mahāprājñaḥ so'śvamedhaphalaṃ bhavet॥21॥
ahorātropavāsena pūjayedyastu bhāskaram।
saptamyāmatha ṣaṣṭhyāṃ ca sūryalokaṃ sa gacchati॥22॥
By fasting on the sixth (Ṣaṣṭhī) day, one is freed from all sins. 17
By taking a single meal (ekāhāra) on the sixth day and worshipping the Sun (Ravi), and also following disciplined vows (niyama-vrata) on the seventh day (Saptamī), one who is devoted to the Sun attains the merit of an Aśvamedha yajña (horse-sacrifice). 20-21
By fasting for a full day and night and worshipping Bhāskara (the Sun) on either the seventh (Saptamī) or sixth (Ṣaṣṭhī) day, one reaches the solar realm (Sūrya-loka) after death. 22
Reference -
Samva Purana translated in Hindi by S.N. Khandelwal
(The preface of the book also mentions about Chhath Puja in detail).
