A lot of users on here simply assume a Eurocentric worldview to everything without doing even basic research on anything. The answers a lot of the time are as ridiculous as reading Afrocentrist people and their clear self bias.
If you are interested in actual history and the reality of the world, instead of wanting to stroke your insecure ego, then the below is the actual history of Japanese days and their calendar.
- The original 7 days originates in ancient Mesopotamia and is based on the 7 lights in the sky (sun, moon, Venus, Saturn, Mars, Mercury, Jupiter). Yes I am sorry it isn't a European invention and Europeans only adopted it from Mesopotamia after spreading to Greece and then Rome and then the rest of Europe.
- This system didn't spread to just Europe, it spread to India, and then from India into China. As you know China had a lot of influence on early Japanese culture, and this is where Japan in turn got the 7 day week from. The writings of the Chinese Scholar Fan Ning in the 300's prove that this system had made it to China by this date. The Chinese used element names in place of the planet names.
- A Japanese monk brought this system back from mainland Buddhists in the year 806. Soon after Japanese astronomers learned of this and worked these 7 days into the Japanese calendar. There is still a surviving diary of a Japanese statesman from the year 1007 who uses this system of dating. It was commonly used in Japan by this time.
The "west" or "Europeans" did not invent this system, nor did they introduce it to Japan. Again I am sorry if real history doesn't stroke your ego, but those are the facts.
So what did Japan adopt during interactions with western countries?
Well Japan already had months, years and naming of the 7 days in a cycle (as did China, where Japan got it from originally).
So what actually was introduced via western contact primarily in the west's heyday in the 19th century was the Gregorian calendar.
All Japanese know the Gregorian calendar today, they still use the original Japanese calendar as well for official duties, yet the Gregorian calendar is actually more common in everyday Japanese life today than the traditional Japanese calendar. It was during the Meiji era that the co-adoption of the Gregorian system was chosen.
The Gregorian calendar itself was a model Europeans made after they themselves adopted the Mesopotamian/Babylonian system. This particular Gregorian calendar was formulated in 1582 by a pope in Italy. Europeans of course long knew of the months and weeks and years because they had known of this Mesopotamian/Babylonian system ever since around the 1st century AD when Rome first adopted it via Greece. The Gregorian calendar is based 99% on this, except it lists dates in relation to Christ as BC and AD.
Yet modern people call these common era and before common era to remove the religious connotations. The Gregorian calendar's months and years did not align to the same times in the Chinese and Japanese calendar. Meaning the original Japanese lunar based system had it's month prior to the Gregorian calendar and thus the years started and ended at different times as well, despite both operating on a 12 month and 7 day system.
Upon adopting the Gregorian calendar, the Japanese used their own traditional 7 day week names in the Gregorian calendar system. And used their traditional month names as well.
Japan and China had always considered Sunday the start of the week, as this is from the ancient Mesopotamian system which they based their own off in ancient times.
The concept of Monday being the first day is a newer concept and Monday is technically considered the first day of the week by international standards. This means that most modern European countries consider Monday the first day of the week.
In Japan the first day of the week is Sunday, like it always has been since the 9th century and also has been in China since the 4th century. And this has nothing to do with the introduction of the Gregorian calendar or Europeans I am afraid.