It is said that the beginning of i'tikaf is after Maghrib, while the text of the hadith is after Fajr, and those who said after Maghrib merely interpreted it, stating that the beginning of Fajr is for preparation, and the original start is after Maghrib.
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The questioner : The questioner says: You state that the beginning of i'tikaf is after Maghrib, and the text of the hadith says after Fajr. Those who said after Maghrib merely interpreted it, saying that the beginning of Fajr is for preparation, and the original start is after Maghrib. We ask for clarification?
The Shaykh : The clarification is that if "the ten days" is mentioned in a general sense, it includes both the days and the nights. It is well known that the first night of the ten days is the setting of the sun on the twentieth of Ramadan. For this reason, the Prophet (peace and blessings be upon him) was shown Laylat al-Qadr on the night of the twenty-first. If we were to say that the one performing i'tikaf who seeks Laylat al-Qadr does not enter until the Fajr of the twenty-first, he would miss the possibility that Laylat al-Qadr is the night of the twenty-first. Their interpretation, as found in Fath al-Bari and other works, is a valid interpretation. Then we say that the Messenger of Allah (peace and blessings be upon him) entered his i'tikaf after Fajr because he did not know that Laylat al-Qadr was in the last ten days until that night, so he began his i'tikaf from Fajr.
The Shaykh : The clarification is that if "the ten days" is mentioned in a general sense, it includes both the days and the nights. It is well known that the first night of the ten days is the setting of the sun on the twentieth of Ramadan. For this reason, the Prophet (peace and blessings be upon him) was shown Laylat al-Qadr on the night of the twenty-first. If we were to say that the one performing i'tikaf who seeks Laylat al-Qadr does not enter until the Fajr of the twenty-first, he would miss the possibility that Laylat al-Qadr is the night of the twenty-first. Their interpretation, as found in Fath al-Bari and other works, is a valid interpretation. Then we say that the Messenger of Allah (peace and blessings be upon him) entered his i'tikaf after Fajr because he did not know that Laylat al-Qadr was in the last ten days until that night, so he began his i'tikaf from Fajr.