Is it impermissible to fast a supererogatory fast on a single day unless it is accompanied by another day, in defiance of the Jews, and is this reason unconvincing?

It is permissible to fast individual days of the week, with the exception of Friday, Saturday, and Ashura. It is disliked to single out Friday and Saturday for fasting. It is recommended to fast the day before Ashura or the day after it. The wisdom behind disliking singling out Friday for fasting is that this day is for supplication, remembrance, and other acts of worship that require energy. The wisdom behind disliking singling out Saturday is to avoid imitating the Jews. As for Ashura, the reason is to differ from the Jews, based on the saying of the Prophet, peace be upon him: "If I live until the next year, I will surely fast the ninth [of Muharram]." A Muslim must submit to the rulings of the Sharia, whether they understand the wisdom behind them or not.