Ibn Ṭāūs in his work al-Iqbāl, cites a tradition reported on the authority of the Prophet (p) he comes across in a work called Fatḥ al-Abwāb ila Dār al-Thawāb which says the following: On the night of 22nd Rajab, whoever performs 8 units of prayers, in which al-Ḥamd is recited one time, Sūrah al-Kāfirūn 7 times, and after its completion one recites the Ṣalawāt upon the Prophet (p) 10 times, and seeks forgiveness from Allah (i.e. says Astaghfirullah) 10 times – they will not leave this world until they see their place in heaven, and their death will be upon the religion of Islām, and their reward will be that of 70 Prophets.
Ibn Ṭāūs goes on to say that Shaykh al-Mufīd in one of his works Ḥadāiq al-Riyāḍ writes that on the 22nd of Rajab, 60 AH, Allah (swt) took the life of one of the greatest Pharaohs of this nation – namely Mu’āwiyah b. Abī Sufyān – and it is recommended to fast on the day, out of gratitude to Allah (swt) on his death.
Source: Iqbāl al-A’māl, vol. 2, pg. 667
Amongst some subcontinent Shī’ī and as well as some Sunnī Muslims, the custom of Kūnḍay takes place on the 22nd of Rajab. The research papers I have looked into discussing the historicity of this tradition agree that the practice began around 1906. The son of the famous poet Amīr Mīnāī (d. 1900 CE), Khurshīd Aḥmad Mīnāī wrote and distributed a book titled Dastān-e ‘Ajīb, which first spread in the city of Rāmpūr, Uttar Pradesh and slowly crept into the city of Lucknow. By 1911 it had spread to villages and cities around Lucknow such as the village of Udh and region of Rohilkhand and subsequently much of the subcontinent.
The book contains a lengthy – and a very obviously fabricated – story about a lumberjack who had gone missing for 12 years while his wife continued to wait for him. As the story goes, on one of the days she overheard heard Imām al-Ṣādiq (a) say that if anyone desires to fulfil their legitimate wishes, then on the 22nd of Rajab they should prepare Pūrīs (a deep-fried bread originating from the Indian subcontinent) with plain flour, then place them in baked clay pots (kūnḍay), recite al-Ḥamd in the name of the Imām and invoke the grace of Allah (swt). God-willing their difficulties will be resolved and their legitimate desires will be fulfilled.
Some – particularly subcontinent Sunnī scholars who have deemed this act an innovation and have asked Sunnī Muslims to stop practicing it – have hypothesized that the story may have been fabricated so that the Shī’as would have reason to celebrate and invoke blessings of Allah (swt) on the 22nd of Rajab as it happens to be the day Mu’āwiyah died. While this is definitely possible, it is hard to say anything with assurance.