The night of Juhani is a blessed night that is mentioned in a number of Ahadith and narrations. It is the eve of the tentey-third of the holy month of Ramadhan, which is expected to be Laylat al-Qadr [night of decree].
The reason for calling it "al-Juhani", as stated by Shaykh Saduq, it is attributed to a man of the companions of the Holy Prophet (Allah's prayers be upon him and upon his holy Household) who is called Abdullah Ibn Unays al-Ansari al-Juhani (Juhayna was a famous tribe), following a discussion that happened between him and the Holy Prophet (Allah's prayers be upon him and upon his holy Household).
It is narrated by Mohamad Ibn Yusuf from his father saying: I heard Imam al-Baqer (peace be upon him) saying: "al-Juhani came to the Prophet (Allah's prayers be upon him and upon his holy Household) and said: "O Messenger of Allah, I have camels, sheep, young children and workers. I would like if you command me of one night in which I come to enter the Masjid to pray in the month of Ramadhan."
The Messenger of Allah (Allah's prayers be upon him and upon his holy Household) called him and told him the answer in his ear [secretly]. Since then, al-Juhani comes on the eve to the twenty-third with his camels, sheep and family to his place."
And in a narration of Abdullah Ibn Bukayr narrating from Zurarah asking either Imam al-Baqer of Imam as-Sadeq (peace be upon both of them): What are the nights of the month of Ramadhan in which the Ghusl [full-body ritual bath] is recommended? He replied: "The eve of the nineteenth, the eve of the twenty-first and the eve of the twenty-third." And he said: " the eve of the twenty-third is the night of Juhani, as he told the Messenger of Allah (Allah's prayers be upon him and upon his holy Household): "O Messenger of Allah, I am a man whose house is far away. Tell me which night I should go out for prayer on.” Then he ordered him to do so on the eve of the twenty-third."