The 7 day week is not really based on western calendar. Firstly, lets look at how classic predictions ( Jyothisha Shastra ) of the Vedas for answers to these questions:
Why do we have only 7 days in a week? Why can't we have 8 or 9? �What is an hour? �Why do we have only 24 hours in a day? Why can't we have 30 or 40 units and call it an hour?
Ancient Indians were so well versed that they often used 4 to 5 different units of time. If you ask your Indian grand mother, she might have told you that during her days, people measured time in a weird unit called ghati/ghadiya (1 ghati = 24 minutes).
1 day is the time lapsed between two sunrises. Sandhi in sanskrit means knot or junction and hence the junction points are named as pratah sandhya(Early morning) and sayam sandhya (Evening) which divide the standard day into two halves i.e from sunrise to sunset and sunset to the next sunrise.
Since there are 12 zodiac constellations, each constellation is assigned a part of the half a day unit and hence 12 parts in half a day each, together 12+12 = 24 units. This is the concept of hora or HOUR. (Yes, hora is the standard hour. English unit of time and Sanskrit unit of time named similarly. Strange coincidence, don't you think so?)
Just as there are constellations associated with each hora, each graha is assigned rulership of individial hora. The order of the planetary rulership of horas is as follows.
1) Surya (Sun/SUN-DAY) followed by
2) Shukra (Venus/FRI-DAY) followed by
3) Budha (Mercury/WEDNES-DAY) followed by
4) Soma (Moon/MON-DAY) followed by
5) Shani (Saturn/SATUR-DAY) followed by
6) Guru (Jupiter/THURS-DAY) followed by
7) Mangala (Mars/TUES-DAY).
In Indian Prediction science (jyothish shastra), the rising sign at the time of sunrise is noted down and is considered very important to make any astronomical/astrological calculations regarding a chart, esp. to find out the janma lagna (Birth time), it is is very essential. So, the rising sign is very important. Likewise, the planetary rulership of the hora during the time of sunrise is noted down. The planet that rules the hora at the time of sun rise is assigned the rulership of the whole day.
The day Ravi-vara (or Sun-day) is named after Ravi/Sun who is assigned lordship of the day because he rules the hora at the time of sunrise of that day. Now, following Sun, the next hour after sunrise is ruled by Shukra followed by the rest. In the above mentioned order of rulership of horas, calculate the next ruling planet that comes after 24 horas, i.e
1st hour by Ravi,
2nd hour by Shukra,
3rd hour by Budha,
4th hour by Soma,
5th hour by Shani,
6th hour by Guru,
7th hour by Mangala,
8th hour by Ravi,
9th hour by Shukra,
10th hour by Budha,
11th hour by Soma,
12th hour by Shani,
13th hour by Guru,
14thth hour by Mangala,
15th hour by Ravi,
16th hour by Shukra,
17th hour by Budh,
18th hour by Soma,
19th hour by Shani,
20th hour by Guru,
21st hour by Mangala,
22nd hour by Ravi,
23rd hour by Shukra,
24th hour by Budha
*End of a day***
25th hour by Soma(moon/Monday)
As you see it turns out that Soma is the ruler of the next day's sun rise. And hence, the next day Soma-vara (or Mon-day) is named after Chandra/Moon who is assigned lordship of the day because he rules the hora at the time of that day's sunrise.
In the same order, Mangala-vara (Tuesday) for Mangala/Mars being the hora ruler at sunrise,
Budha-vara (Wednesday) for Budha/Mercury being the hora ruler at sunrise,
Guru-vara (Thursday) for Deva Guru Brihaspathi/Jupiter being the hora ruler at sunrise,
Shukra-vara (Friday) for Shukra/Venus being the hora ruler at sunrise,
Shani-var (Satur-day) for Shani/Saturn being the hora ruler at sunrise,
Now after Saturday, the cycle reverts to
1) with Surya being the ruler of the hora at the time of next day's sunrise. This is the reason why there are only 7 days in a week based on these calculations of hora and their planetary rulership as mentioned in the vedic texts.
One may be a Christian, Muslim, Sikh or Jew, knowingly or unknowingly they follow the methods of the ancient Indian Rishis.
This is the reason why Hindu dharma is called Ancient Indian Civilisation Principles-ie called as Sanatana dharma (i.e eternal and expansive in its very nature).
It is funny how the whole world believes that the 7 day week is a western concept!