According to the ISO 8601(International Organization for Standardization), Sunday is considered the last or the seventh of the week. Even for Christians and according to the Bible, God created the earth and everything in it, in six days, and on the seventh day, he rested. As it was done, it is mentioned in the book of Exodus, "Remember the sabbath day and keep it holy which means no work will be done on Sunday and it will be a day of the Lord, to praise, honor and glorify him. Thus Sunday was known as the day of the Lord. So for Christians, The first day is Monday
In Islam, the communal prayer is done on Friday and they have Friday as an off day. And in Eastern or Muslim countries, they have Friday and Saturday as holiday and their first day of the week begins on Sunday.
According to Hinduism or Vedas, Each day is known by the name of the planet. As we have 9 planets, two are known as shadow planet and the other Seven is the name of the weeks. As Sun is considered the omen that supports life on earth is given the prime importance, the first day is named after it.
Nowadays, the first day depends upon the company or the industrial zone that it falls or your work is considered. When I was working in Mumbai in the industrial area, we had weekly off on Friday due to a power cut. That time, our first day was Saturday. After shiting to Deonar, we had weekly off on Sunday, so my first day was Monday. Now, I have weekly off on Friday and Saturday, so my first day is Sunday. The first day changes with the situation, region, or country.
“The most important thing in life is to learn how to give out love, and to let it come in." — Morrie Schwartz