ISO Week Numbering vs. the US Convention: What's Different
ISO 8601 weeks start on Monday and week 1 is defined by containing the year's first Thursday. A commonly used US convention instead starts weeks on Sunday and often simply treats the week containing January 1st as week 1 — the two conventions can produce different week numbers for the same date, especially near the beginning and end of the year.
Neither convention is more "correct" in an absolute sense — they're simply different standards developed for different contexts, and knowing which one is in use avoids confusion.
Week-start day
ISO 8601 treats Monday as the first day of the week, aligning with common international business and calendar conventions in much of the world. The common US approach treats Sunday as the first day of the week, which shifts which specific days group together into the same numbered week.
Where week 1 begins
ISO 8601's "first Thursday" rule (covered in the ISO week guide) can place early January days in the previous year's final week. The simpler US convention of treating the week containing January 1st as week 1, regardless of how the days split across years, generally avoids that specific quirk but produces different numbering as a trade-off.
Why it matters to know which is in use
For anything involving cross-team or cross-country coordination using week numbers (a common practice in manufacturing, logistics, and some business planning), confirming which specific convention is being referenced avoids a real, if usually minor, miscommunication about which actual calendar dates a "week 12" or "week 45" refers to.