Well after a certain research and with the helpful answer of our brother @ali-علی I found out that all weekdays have been known with other names in the Jahilya which if found very helpful and liked to share with you, even if i couldn't exactly find out at what time (exactly) the names were changed:
The poet and linguist of the 'Abbasid Dynasty ibn Daryd said referring to the days of a week:
أؤمّل أن أعيش وأنّ يومي ** بأوّل أو بأَهْوَن أو جُبَار
أوالتالي دُبَار أو فيومي ** بمُؤْنِس أو عَرُوبَة أو شِيَار
- Sunday: now known as al-Ahad الأحد had a similar name as it was named أول Awwal = first!
- Monday: was known as أهون Ahwan or as some said أهوية Ahwiya or أَوْهَد Awhad.
- Tuesday: was known as جُبَار Jubar (Jobar).
- Wednesday: was known as دُبَار Dubar (Dobar) or وبار wabar
- Thursday: was known as مُؤْنِس Mu'nis (Mo'nis)
- Friday: was known as عَرُوبَة 'Arubah ('Arobah).
And it was pretended that the first who have named it Jumua' was كَعْبُ بْنُ لُؤَيٍّ Ka'an ibn Lu'ay (Lo'ay) an ancestor of the Messenger of Allah (peace be upon him) (7.th Grandfather الجد السابع).
But as-Suhayli السُّهَيْلِيُّ in his book ( الرَّوْضِ الْأُنُفِ ar-Rawd al-Unuf) said that Ka'ab ibn Lu'ay was the first who gathered (Quraish) on 'Arubah, but it only was named Jumua' when Islam came!
Tha'lab ثَعْلَبٌ said: Jumua' was named so because Quraish used to gather with Qusay قُصَيٍّ in dar an-Nadwa دَارِ النَّدْوَةِ!
- Saturday: was known as شبار Shibar or شِيَار Shiyar.
For more details see here (in Arabic) or a fast overview here (in Arabic)
Why was the 7th day in the Week related to the number 6?
Saturday was called sabt because it was the day where Allah began his creation at ... this is the main opinion.
There's a Hadith and another narration of ibn 'Omar (May Allah be pleased with both) with differences
عَنْ عَبْدِ اللَّهِ بْنِ عُمَرَ ، قَالَ : خَلَقَ اللَّهُ التُّرْبَةَ يَوْمَ السَّبْتِ ، وَخَلَقَ الْحِجَارَةَ يَوْمَ الْأَحَدِ ، وَخَلَقَ السَّحَابَ يَوْمَ الِاثْنَيْنِ وَخَلَقَ الْكُرُومَ يَوْمَ الثُّلَاثَاءِ ، وَخَلَقَ الْمَلَائِكَةَ يَوْمَ الْأَرْبِعَاءِ ، وَخَلَقَ الدَّوَابَّ يَوْمَ الْخَمِيسِ ،
وَخَلَقَ آدَمَ يَوْمَ الْجُمُعَةِ فِيمَا بَيْنَ الْعَصْرِ وَغُرُوبِ الشَّمْسِ
Which seems to explain the creation steps, but both don't mention any creation of sky or heaven etc.
Sabt (saturday) also refers to whole week for example in this hadith as we see from this translation "فَلاَ وَاللَّهِ مَا رَأَيْنَا الشَّمْسَ سَبْتًا" ="By Allah, we did not see the sun throughout the week." BTW the same applies for jum'ah in some local Arabic dialects, for example we haven't seen each other for a week may be expressed by we haven't seen each other for a jumu'ah!
Many Ahadith refer to Jumua' as a special day see for example in Sahih Muslim in Sunan abi Dawud in Sunan an-Nasa-i or in Jami' at-Tirmidhi
And Allah knows best!