# Calculate Week Number from Date

If you need to calculate the week number from a given date, there are several ways to accomplish this in programming. One common method is to use the `ISO 8601` standard, which defines the first week of the year as the week containing at least four days in January. This means that the first week of the year may start on either December 29, 30, 31, or January 1, depending on the year and calendar.

To calculate the week number using `ISO 8601`, you can use the formula:

``````week_number = (date - first_thursday_of_year + 3) // 7
``````

Where `date` is the date you want to calculate the week number for, and `first_thursday_of_year` is the date of the first Thursday of the year (which can be found using the same formula).

Another method to calculate the week number is to use the `%U` or `%W` format specifier in programming languages such as Python or C. The `%U` specifier defines the week as starting on Sunday, while the `%W` specifier defines the week as starting on Monday.

For example, in Python, you can use the `strftime` function to format a date string with the `%U` specifier:

``````import datetime

date_string = "2021-08-26"
date = datetime.datetime.strptime(date_string, "%Y-%m-%d")
week_number = date.strftime("%U")

print(week_number)  # Output: 34
``````

This will calculate the week number for the given date, assuming that the week starts on Sunday.

In conclusion, there are several ways to calculate the week number from a given date, depending on the programming language and standard you are using. By using the `ISO 8601` standard or the `%U`/`%W` format specifier, you can easily calculate the week number and use it in your application as needed.

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