It is disliked for the sake of purity; because it resembles non-Muslims and we have been prohibited from it, and because it involves glorifying days that we have been forbidden to glorify. If it coincides with a day that one is already fasting, then there is no harm, as mentioned in Badā'iʿ al-Ṣanāʾiʿ 2: 79, Ḥāshiyat al-Tabyīn 1: 332, and Majmaʿ al-Anhār 1: 254.