Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.150.
We have already mentioned that the voluntary i’tikaf does not have any specific time period. Whenever a person enters the mosque and makes the intention of becoming closer to Allah by staying there, he will be performing i’tikaf until he leaves. If he has the intention to perform i’tikaf during the last ten days of Ramadan, he should begin it before the sun sets. Al-Bukhari records from Abu Sa’id that the Prophet, upon whom be peace, said: “Whoever makes i’tikaf with me is to make i’tikaf during the last ten [nights].” The ten refers to the last ten nights which begin on the night of the 20th or the 21st.
Concerning the statement that when the Prophet desired to make i’tikaf he would pray the morning and then go to the place of his i’tikaf, it means that he used to enter the place which he had prepared for his seclusion, but the actual time that he entered the mosque for his seclusion was during the beginning of the night.
According to Abu Hanifah and ash-Shafi, whoever performs i’tikaf during the last ten days of Ramadan must leave the mosque after sunset on the last day of the month. Malik and Ahmad say that it is acceptable to leave after sunset, but they prefer for the person to remain in the mosque until the time for the ‘id prayer.
Al-‘Athram records from Abu Ayyub that Abu Qulabah would stay in the mosque on the night before the ‘id prayer and would then go to the ‘id prayer. During his i’tikaf, he had no mat or prayer carpet to sit on. He used to sit like anyone else. Abu Ayyub said: “I came to him on the day of ‘id and on his lap was Juwairiyah Muzinah. I thought it was one of his daughters, but it was a slave that he had freed, and he came that way to the ‘id prayer.” Ibrahim said: “The people preferred that one who performed i’tikaf during the last ten days of Ramadan stay in the mosque on the night of ‘id and then proceed to the ‘id prayer from the mosque.
If an individual makes a vow to perform i’tikaf for a specific period of days, or he wants to do so voluntarily, then he should begin his i’tikaf before dawn and leave when all the sun’s light has gone, regardless of whether that be during Ramadan or at another time. If he vowed to perform i’tikaf for a night or a specified number of nights, or if he wants to do so voluntarily, then he should begin his i’tikaf before the sun has completely set and may leave when it is clear that dawn has begun. Ibn Hazm says: “The night begins when the sun sets and ends with dawn. The day begins with dawn and is completed by sunset. This is not a condition upon anyone unless he desires or intends to fulfil it. If one vows or wants to make i’tikaf voluntarily for a month, he should begin during the first night of the month. He should enter the mosque before the sun has completely set and may leave after the sun has completely set at the end of the month – regardless of whether it is Ramadan or otherwise.”