When does the one in i'tikaf (spiritual retreat in the mosque) enter his place of retreat, and when does he leave it?

Question : Peace be upon you, and Allah's mercy and blessings. Honorable Shaykh! I love you for the sake of Allah, and I hope you will answer the question: When does the one in i'tikaf enter his retreat? And when does he leave?

Answer : Yes. In the first sentence, I ask Allah, in Whom I love you, that you love Him and that He make us all among the close friends of Allah (Mighty and Majestic) and the awliya (close friends of Allah). The one in i'tikaf enters his retreat—meaning he enters the mosque in which he intends to perform i'tikaf—when the sun sets on the night of the twentieth of Ramadan, and he leaves when the sun sets on the first night of Shawwal.
Because the hadiths are abundant that the Messenger (peace and blessings be upon him) would perform i'tikaf in the last ten days of Ramadan. It is known that Ramadan ends with what? With the entry of the month of Shawwal, meaning the night of the festival, Ramadan ends. When do the last ten days of Ramadan begin? They begin from the night of the twenty-first, not from the dawn of the twenty-first.
As for the hadith of 'Aishah, established in al-Bukhari, that the Messenger of Allah (peace and blessings be upon him) prayed the Fajr prayer and then entered his retreat, the meaning, as Ibn Hajar said, is that he had a specific retreat in which he secluded himself from the people. It does not mean entering the mosque, for he had already entered the mosque from the beginning of the night. But he secluded himself in this specific retreat from the people from the morning of the twenty-first. Yes.