ISO Week System
The ISO week system is based on the ISO 8601 standard used by the WHO.
Overview
The ISO week system is part of the ISO 8601 standard issued by the International Organization for Standardization. It is used by the World Health Organization (WHO) for disease reporting and surveillance. The first day of any ISO week is Monday. Week numbering is sequential beginning with 1 and incrementing with each week to a maximum of 52 or 53.
Geographic Usage
The ISO week system aligns with countries where the week starts on Monday. This includes most of Europe (United Kingdom, Germany, France, Russia), parts of Asia (China, Thailand), and Oceania (Australia, New Zealand).
Week 1 Definition
ISO week 1 is the week containing the first Thursday of the year. Equivalent definitions include the week containing January 4, or the week starting with the Monday nearest to January 1. Based on this rule:
- When January 1 occurs on Monday–Thursday, it is in ISO week 1
- When January 1 occurs on Friday–Sunday, it is in the last ISO week of the previous year (week 52 or 53)
- Consequently, December 29–31 may fall within ISO week 1 of the following year
53-Week Years
Some years have 53 ISO weeks instead of 52. Examples include 2020 and 2026.
Comparison with CDC System
Key differences from the CDC (MMWR) system:
- ISO weeks start on Monday; CDC weeks start on Sunday
- Week 1 definitions differ, so the same date may fall in different week numbers
- The week containing January 1 may differ between systems