Isn't the Prophet's—peace and blessings be upon him—habit of visiting al-Baqi' every time it was 'Aishah's—may Allah be pleased with her—night, and saying: "Peace be upon you, O abode of a believing people. What you were promised has come to you, deferred until tomorrow; and indeed, if Allah wills, we shall join you. O Allah, forgive the people of Baqi' al-Gharqad," considered specifying a day or time for visiting graves, even though scholars disapprove of that?

The Hadith (definition) you mentioned is authentic and was narrated by Imam Muslim, with the wording from Aisha: "Whenever it was her night with the Messenger of Allah, may Allah bless him and grant him peace, he would go out at the end of the night to Al-Baqi' and say: 'Peace be upon you, abode of believing people. What you were promised has come to you. You are postponed until tomorrow, and indeed, Allah willing, we shall join you. O Allah, forgive the people of Baqi' Al-Gharqad.'"

This Hadith does not indicate specifying the visit to a particular night. For Aisha's turn was not on a specific day, but rather it was a continuous practice of the Prophet, may Allah bless him and grant him peace, during her turn. Visiting graves is recommended without specifying a day or time, and it is not permissible to designate a specific day for visiting cemeteries due to the lack of evidence for it. Designating it to a specific day with the belief of its virtue is an innovation (bid'ah). Its permissibility is whenever it is facilitated, without specific designation.