I say, and with God's success: Whoever vows to perform I'tikaf for days is obligated to do so for the nights as well, and likewise, if one vows to perform I'tikaf for nights, they are obligated to do so for the days in succession without condition; because mentioning days in the plural includes the corresponding nights, and mentioning nights includes the corresponding days. Allah, the Exalted, said: {Three days except in a sign} Al-Imran: 41, and He, the Exalted, said: {Three nights in succession} Maryam: 10. The story is the same, expressed sometimes in days and sometimes in nights, thus it is known that mentioning one in the plural encompasses the other. The first night is included if it is consecutive, even if succession is not stipulated, and the I'tikaf begins before sunset on the first night and ends after sunset on the last day, unlike if one intended the days specifically for daytime, in which case their intention is valid; because it is the literal meaning of their words. Refer to: Al-Wiqayah p. 245, Al-Mabsut 3: 122, Al-Tabyin 1: 353, and Allah knows best.