What "day of year" means
The day of year (also called the ordinal date) is the sequential count of days since January 1st — it ranges from 1 to 365 in common years, and 1 to 366 in leap years.
What the ISO 8601 week is
The international ISO 8601 standard defines weeks numbered 1 to 52 or 53, always starting on Monday. Week 1 of the year is the one containing the first Thursday of the year — this rule avoids ambiguity around year boundaries.
Difference from the common calendar
Unlike the US format (week starting on Sunday), the ISO standard is the one most widely used internationally in project management systems, contracts and corporate planning.
Frequently asked questions
Why can a year have 365 or 366 days?
Leap years have one extra day (February 29th), totaling 366 days, to compensate for the fact that Earth's orbit isn't an exact number of days.
Does the ISO week always start on Monday?
Yes, that is one of the core rules of the ISO 8601 standard, unlike the US calendar, which starts the week on Sunday.
Can an ISO week belong to two different years?
Yes — dates in late December or early January can, under the ISO rule, belong to the last week of the previous year or the first week of the next.