As there is some confusion between Sunday and Monday. Kindly tell me how to explain when someone ask me this question, that which is the first day of week ?
There's a lot of confusion here, and a dozen theories.
One of those points out that the Bible has Jesus rising on the first day of the week (i.e. Easter Sunday), which is in keeping with the Old Testament tradition (God spent six days creating the world, and rested on the seventh day, Sabbath, known today as Saturday).
Another one suggests that over in England, the Puritans were in favor of "returning" to some of the more austere Old Testament customs, stressing the importance of works (in the biblical sense) and the letter of the Mosaic Law (rather than the spirit of said Law, as both Jesus and St. Paul recommended). This necessitated designating Saturday as the last (seventh) day of the week, the day of rest. The sentiment was then exported to the New World by folks who found some of the Puritan ideas congenial to them.
It might be worth noting that in tongues belonging to very different language groups (such as Icelandic (North Germanic), Finnish (Baltic Finnic), and Russian (Slavic), the name for Wednesday means, literally, "middle of the week" (respectively, Keskiviikko, Miðvikudagur, and Среда). One might be tempted to point out that Wednesday is, in fact, the middle of the work week; which would not be quite accurate since, in all of the above languages, the name appeared long before the institution of the two day weekend (a very recent development, dating back merely to the beginning of the 20th Century).
ISO-8601 clearly decree that Monday is the fist day of the week, which should end all doubt.
Monday is the first day in the whole world. ISO standard clearly says so.. The fact that some countries CONSIDER Sunday or Saturday the first day, doesn't alter the fact that Monday is the first day by decree from ISO!