Allah U is directing this speech to the Jews who were in the Arabian Peninsula in Al-Madinah Al-Munawarrah during the time of revelation.
The names of only two days of the week are mentioned in the Holy Qur’an – Friday and Saturday.
The word Friday was mentioned in the Surah Al-Jumu’ah:
﴾يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا نُودِيَ لِلصَّلَاةِ مِن يَوْمِ الْجُمُعَةِ فَاسْعَوْا إِلَىٰ ذِكْرِ اللَّهِ وَذَرُوا الْبَيْعَ﴿
“Oh you who have believed, when [the adhan] is called for the prayer on the day of Jumu’ah (Friday), then proceed to the remembrance of Allah and leave trade. That is better for you, if you only knew.” [Surah Al-Jumu’ah: 9]
And the word Saturday was mentioned here and in Surah Al-A’raaf.
The days of the week are 7, starting with Sunday, Monday, Tuesday, Wednesday, Thursday, and the 6th is Friday and the 7th is Saturday. And the word Jumu’ah and Sabt are words that are not derived from numbers nor connected to them. The scholars differ as to what caused these words to develop.