The ḥadīth masters take this report as clear evidence that fasting on the disbelievers’ festival day is recommended precisely in order to differ from them.
It is likewise narrated from ʿĀʾishah (may Allah be pleased with her) that she said:
“The Messenger of Allah (peace and blessings be upon him) used to fast during one month on Saturday, Sunday, and Monday, and during the next month on Tuesday, Wednesday, and Thursday.”
Al-Tirmidhī transmitted this ḥadīth and remarked, “A ḥasan (good) report.” He added, “Ibn Mahdī related it from Sufyān without ascribing it to the Prophet.”
These two reports, however, are not a conclusive proof against those who deem fasting Saturday by itself disliked. Their rationale is that the Jews abstain from work on that day, and fasting may be viewed as part of that abstention. When one fasts Saturday together with Sunday, the objectionable singling-out disappears, and the desired opposition to them is still achieved by fasting on their day of rest.