Fasting on the day of doubt (yawm al-shakk) — that is, the 30th day of Shaʿbān when the start of Ramadan is uncertain — has been prohibited by the Messenger of Allah (peace and blessings be upon him). Therefore, it is not permissible (caiz değildir) to fast on this day.
Several wisdoms may underlie this prohibition: to ensure that Muslims enter the month of Ramadan in a state of strength and vitality, to encourage careful determination of the beginning and end of acts of worship, and to prevent the habitual confusion of Ramadan’s start through premature fasting.
In this regard, the following hadith is reported:
“Whoever fasts on the day of doubt has opposed Abū’l-Qāsim (the Prophet, peace and blessings be upon him).”
(al-Bukhārī, “Ṣawm,” 11; al-Tirmidhī, “Ṣawm,” 3; al-Nasā’ī, “Ṣiyām,” 37)