Two ways to number weeks
There is no single answer to "what week is it" because two conventions are in common use. The ISO 8601 standard, used in Europe, in logistics and manufacturing, and in most software, starts every week on Monday and defines week 1 as the week that contains January 4 (equivalently, the first Thursday of the year). Weeks are always seven full days, so a year has exactly 52 or 53 ISO weeks, and the first few days of January can belong to week 52 or 53 of the previous year while December 29 to 31 can be week 1 of the next. For example, January 1, 2027 is a Friday and falls in ISO week 53 of 2026.
The US convention, which is what Excel's WEEKNUM function returns by default and what most American wall calendars show, starts weeks on Sunday and calls the week containing January 1 week 1, regardless of how short it is. Under this system there can be 53 or even 54 partial weeks in a year and the numbers can differ from ISO by one, occasionally by a whole week around New Year. This tool shows both, with the date range each week covers, so you can match whichever system your employer, school or supplier uses.
How ISO week numbers are computed
The algorithm shifts the date to the Thursday of its week (Thursday always lies in the correct ISO year), takes that year, then counts the weeks from the Thursday of week 1. A year has 53 ISO weeks when January 1 is a Thursday, or when it is a Wednesday in a leap year; 2026 has 53 weeks and so does 2032. The ISO date notation puts it all together as YYYY-Www-D, for instance 2026-W40-3 for a Wednesday in week 40.
In spreadsheets, use ISOWEEKNUM(date) in Excel or Google Sheets for the ISO number and WEEKNUM(date) or WEEKNUM(date,1) for the US number. In Python, date.isocalendar() returns the ISO year, week and weekday. When two teams report different week numbers for the same delivery date, the difference in convention is almost always the cause.
Day of the year and days remaining
The day-of-year count (sometimes called the ordinal date or, loosely, the Julian day) numbers January 1 as day 1 and December 31 as day 365, or 366 in a leap year. A year is a leap year when it is divisible by 4, except century years, which must be divisible by 400: 2024 and 2028 are leap years, 2100 is not. Days remaining is the length of the year minus the day number, so on day 300 of a common year there are 65 days left. Quarters follow the calendar: Q1 is January through March, Q4 is October through December.