The WEEKNUM function in Excel returns the week number of a given date. The week number is the number of weeks since the first week of the year, which is the week that contains January 1. The first week of the year is considered to be week 1. If the date you supply to the WEEKNUM function is in December, and the year is set to start in January, then the WEEKNUM function will return 53 for that date. You can use the WEEKNUM function to calculate things like how many weeks have passed since a certain event occurred.
To use the WEEKNUM function, you supply a date value as the first argument. You can supply a date value as a string in quotation marks, or as a reference to a cell that contains a date. You can also use the DATE function to create a date value that can be used as an argument for the WEEKNUM function. The second argument for the WEEKNUM function is optional, and it allows you to specify on which day of the week the week should start. The default value for this argument is 1, which means that the week starts on Sunday. If you want the week to start on Monday, you would use 2 as the second argument. You can also use 3 to indicate that the week should start on Tuesday, 4 to indicate that it should start on Wednesday, 5 to indicate that it should start on Thursday, 6 to indicate that it should start on Friday, or 7 to indicate that it should start on Saturday.
Here are some examples of how you might use the WEEKNUM function: