I say, and with Allah's success: Whoever wakes up intending to fast voluntarily, or without the intention of fasting, then says: I have an obligation to perform I'tikaf today, it is not valid, even if it is a time when the intention of fasting is valid; due to the lack of encompassing the day with I'tikaf, and the lack of encompassing it with obligatory fasting. Refer to: Al-Hadiyya Al-Ala'iyya, pp. 185-186, and Allah knows best.