I say, and with Allah's success: Whoever vows to perform I'tikaf for nights is obligated to do so for those days, and likewise, whoever vows to perform I'tikaf for days is obligated to do so for the corresponding nights, consecutively without condition; because mentioning the days in the plural form includes the corresponding nights, and likewise mentioning the 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 a row} Maryam: 10. The story is the same, expressed sometimes in terms of days and sometimes in terms of nights, thus it is understood that mentioning one in the plural form encompasses the other. Refer to: Al-Wiqayah p. 245, Al-Mabsut 3: 122, and Al-Tabyin 1: 353, and Allah knows best.