What is the ISO week date system?
Quick Answer: The ISO week date system counts weeks starting on Monday and labels each week by the year that contains its Thursday. Week 1 is the first week with at least four days in the new year, making this system consistent and widely used in business and international contexts.
The ISO week date system helps avoid confusion by standardizing how weeks are numbered. Instead of starting weeks on Sunday, it begins on Monday, which aligns better with many countries' workweeks.
Week 1 is special because it’s defined as the week containing January 4th, or equivalently, the week with the first Thursday of the year. This means some days at the start or end of January can belong to the previous or next year's week numbering.
This system is part of the ISO 8601 standard and is popular in Europe and in international business for clear scheduling and reporting.