What is the first day of the week and why, explain?

In most cultures Sunday is the first day of the week, although some observe Monday as the first day.

"According to the Bible, God created the Earth in six days, and rested on the seventh day, the Sabbath, i.e. Saturday. This made Sunday the first day of the week, while Saturdays were sanctified for celebration and rest. After the week was adopted in Early Christian Europe, Sunday remained the first day of the week, but also gradually displaced Saturday as the day of celebration and rest, being considered the Lord's Day. In some places Sunday thus came to be viewed as the last day of the week.

"The variation is evident from names of the days in some languages — in Hebrew, Arabic, Greek, Church Latin and Portuguese, some days are simply called by their number starting from Sunday, e.g. Monday is called 'Second day' etc. In other languages, like Slavic languages, days are also called after their ordinal numbers, but starting from Monday, making Tuesday the 'Second day.'"

http://en.wikipedia.org/wiki/Days_of_the_week#First_day_of_the_week

The Anglo-Saxon business model would dictate that the working week starts on a Monday. It's all about interpretation, I suppose.

Just look at your calendar to see which day is the first one of the week. There's a simple reason: custom, or as Reb Tevye would say, "Tradition!"

My Microsoft calendar on Windows starts on a Monday.

My Microsoft Windows calendar (which I never use) starts on Sunday.

https://support.office.com/en-gb/article/Change-dates-on-a-calendar-2f6f9f4f-3160-418c-8305-489911089fc7

And here's the scoop:
"The day of the week that the calendar starts on is controlled by the Regional Options language setting on your computer. With the English (United States) setting, the calendar week starts on Sunday. For the English (United Kingdom) setting, the calendar week starts on Monday."

"The seven-day week as we understand it was, if not originated, at least definitively justified by the Jews. Genesis 2:3 points out that the seventh day was the day on which God rested from creation, and the Law of Moses mandates that the seventh day is to be a day of rest for all - including slaves and even animals (Deuteronomy 5:13-14). In Jewish tradition, Saturday is the Sabbath, and certain Christian bodies keep this strict seventh-day sabbatarianism (notably the Seventh-Day Adventists)."

"But (as Christians believe) Christ was raised on the first day of the week (Matthew 28:1; Mark 16:1-2; Luke 24:1; John 20:1), after the Sabbath was over. Because of this, Christians traditionally have their primary worship events on Sunday, and the Mosaic sabbatarian regulations were gradually transferred to Sunday among the vast majority of Christians."


http://www.madsci.org/posts/archives/sep99/938455019.Sh.r.html