First of all, it should be clear that i`tikaf (spiritual retreat in the mosque) is an act of worship highly recommended for Muslims to perform during the last third of Ramadan. It is of great merit and significance in Islam.
The eminent Muslim scholar, Dr. `Abdul-Fattah `Ashoor, Professor of the Exegesis of the Qur’an at Al-Azhar Univ., states: I`tikaf is a Sunnah. It can be offered at any time by day or night. There is no limit to its time. The Prophet (peace and blessings be upon him) offered i`tikaf in Ramadan as well as in Shawwal. This is proof of the permissibility of i`tikaf during Ramadan as well as all other months of the year. So, whoever is able to offer it during the whole month of Ramadan will have abundant reward from Allah.
There is a hadith that states: “Whoever offers i`tikaf, Allah lays three barriers between him and the Hell-fire…” (Reported by Al-Bukhari). Whoever finds time to offer i`tikaf during the whole month of Ramadan and continues to do so every year will have great reward in sha’ Allah. We cannot consider this as bid`ah (innovation), for bid`ah is something that has no ground in religion, but i`tikaf has ground in Islam.
Based on the above Fatwa, it is clear that there is nothing wrong if someone offers i`tikaf during the whole month of Ramadan.