ISO Week vs Fiscal Week
ISO 8601 weeks start on Monday; US retail fiscal weeks start on Sunday. They diverge by one day and produce different week numbers.
ISO 8601 definition
An ISO 8601 week starts on Monday and ends on Sunday. ISO week 1 of a year is the week that contains the first Thursday of the year — equivalently, the week containing January 4. This produces a week-numbering scheme that always covers exactly 52 or 53 weeks per year and aligns cleanly across the European business calendar.
US retail fiscal-week definition
The US National Retail Federation fiscal calendar starts each fiscal week on Sunday. Fiscal week 1 of a fiscal year is the first Sunday-to-Saturday week of the fiscal year. This matches the US retail convention that weekends are the dominant selling period and a "week" should be bounded by the close of the weekend it contains.
Practical consequence for cross-border reporting
A multinational retailer with European operations on the ISO calendar and US operations on the NRF calendar will see their "week 26" land on different dates and contain different days. Consolidating financials at the week level requires either picking one convention and converting the other, or reporting at the month or quarter level where the conventions reconcile.
When the difference matters
ISO 8601 week numbering starts each week on Monday and assigns week 1 as the week containing the first Thursday of the year. Most European banking, manufacturing, and supply-chain systems use ISO weeks. US retail fiscal weeks start on Sunday and follow the NRF 4-5-4 calendar pattern, where week 1 is the week containing the first Sunday after Christmas of the prior year.
A purchase-order line item dated "WK 14" in a European ERP and the same code in a US retail planning system can refer to dates two days apart, in different fiscal months, sometimes in different fiscal quarters. Cross-border supply contracts must specify which week numbering convention is operative; lazy contract drafting causes preventable disputes at quarter-end.
How to convert
For ISO week ↔ NRF retail week conversion, build a lookup table at the start of the fiscal year and use it for the entire planning horizon. The offset between the two systems is one day for the start of the week and zero, one, or two weeks for the year boundary depending on which weekday January 1 falls on. Good ERP systems let you store both numbers on every transaction; treat the second as a derived field, not a manually entered code.