When does the time for i'tikaf (spiritual retreat in the mosque) begin in Ramadan?

Question:
Does the one performing i'tikaf (spiritual retreat in the mosque) begin from the dawn of the twentieth day or the night of the twentieth?

Answer:
He enters his place of i'tikaf (spiritual retreat in the mosque) at the dawn of the twenty-first day. If he stays the night in i'tikaf (spiritual retreat in the mosque), there is nothing wrong with it. However, Aisha said: "The Prophet (peace and blessings be upon him) would enter his place of i'tikaf (spiritual retreat in the mosque) after praying Fajr (the dawn prayer)," meaning: from the twenty-first day.
Q: Is Laylat al-Qadr (the Night of Decree) not the night of the twenty-first?
A: Yes, it may be the twenty-first, and it may be the twenty-third. It moves within each ten-day period of Ramadan. In the time of the Prophet (peace and blessings be upon him), it occurred on the night of the twenty-first.
Q: Does the one performing i'tikaf (spiritual retreat in the mosque), O Shaykh, enter, for example, after Fajr (the dawn prayer)?
A: He prays with the Muslims in the mosque, even if he has not entered the place of i'tikaf (spiritual retreat in the mosque).