Sorry, this is better.
http://www.wordiq.com/definition/ISO_8601
ISO 8601 standard weeks run from Monday (day 1) through Sunday (day 7). ISO 8601 assumes that Monday is the first day of the week, a practice that is commonly followed in Europe. The USA commonly assumes that Sunday is the first day of the week (following a tradition derived from ancient Jews and ancient Egyptians)
For the week numbering it is relevant to note that in this system Thursday is the day in the middle of the week, even though the Wednesday is in the middle of a working week from Monday through Friday.
Weeks are numbered within a year from 1 to 52 or 53. A complete week that spans two year numbers will adopt the year number and week number from the portion of the week that has the greater number of days within either of the two years. Equivalently, the Thursdays of a year determine the week numbering: week 1 is defined as the week that contains the first Thursday of the year, etc.
In some European business contexts dates are not specified by month and day of the month but by this week number and the day of the week.