Allah, most Majestic, most Glorious, says, “You must complete the term.

Allah, most Majestic, most Glorious, says, “You must complete the term.” (2:181) Shawwal is twenty-nine days, Dhu al-Qa’dah is thirty days because of the words of Allah, most Majestic, most Glorious, “We made an appointment with Musa (Moses) for thirty days and We completed it with ten more days so the appointment with his Lord lasted for forty days.” (7:142) Dhu al-Hajjah has twenty-nine days and Muharram has thirty days, then the other months thereafter, one is complete and the next incomplete.’”

شهور - Allamah Baqir al-Majlisi (Mirʾāt al-ʿUqūl fī Sharḥ Akhbār Āl al-Rasūl (16/232))