See Installing Macros.
You need to create a macro and insert this code.
You would want a field in your document something like: { DocVariable Date \@ "dddd, d MMMM yyyy" }.
The field braces {} cannot be typed, they must be inserted by Word using Ctrl+F9 or by having Word create the field.

For more, see Date Calculation Tutorial, especially the introductory material on creating fields.