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From Excel's own Help Files (Press F1, type in WeekNum):

If this function is not available, and returns the #NAME? error, install and load the Analysis ToolPak add-in.

How?

1) On the Tools menu, click Add-Ins.
2) In the Add-Ins available list, select the Analysis ToolPak box, and then click OK.
3) If necessary, follow the instructions in the setup program.