Enter any date and this tool reports several calendar facts at once. The headline figure is the ISO 8601 week number: under that standard weeks start on Monday, and week 1 is the week containing the year's first Thursday — equivalently, the week holding January 4th. That rule means the first days of January can belong to week 52 or 53 of the previous year, and late December can fall into week 1 of the next, which is why the tool also shows the ISO week-year that the week actually belongs to.
Alongside the week number you get the day of the year (1 to 365 or 366), the day of the month, the weekday name, and how many days remain until year-end. Dates are handled as local calendar days with no time component, so time zones never shift the result by a day. It defaults to today, everything runs in your browser, and nothing is uploaded.