Praise be to Allah, and may Allah’s peace and blessings be upon the Messenger of Allah, his family and his companions. Wa alaikum assalam wa rahmatullahi wa barakatuh. As to what follows: In response to your question, we say: It is permissible to single out Saturday for fasting because the hadith which prohibits that is unreliable and contradicts the authentic hadiths which indicate that voluntary fasting on Saturday is permitted as was pointed out by Imam Ahmad. The most famous of those hadiths is the Prophet’s (peace be upon him) statement to Juwairiah when she told him that she was fasting on Friday. He asked her, “Did you fast yesterday?” She answered, “No”. He then asked her, “Will you fast tomorrow?” She answered, “No.” He then ordered her to break her fast” [al-Bukhari (1986)]. [This hadith indicates that fasting on Saturday is permitted because the Prophet (peace be upon him) asked Juwairiah if she was going to ask on Saturday].
Dr. Khalid al-Mosleh