If you wish to supply the name of the file include these two lines of code in your HTML document:
<!--#config timefmt="%A %B %d, %Y" -->
This file last modified <!--#flastmod file="ssi.shtml" -->
Of course, you will need to replace the ssi.shtml with the actual name of the file that you're referring to. This can be inconvenient if you're just looking for a generic piece of code that you can paste into any file, so you probably want to use the LAST_MODIFIED variable instead, then insert these two lines of code into your document:
<!--#config timefmt="%D" -->
This file last modified <!--#echo var="LAST_MODIFIED" -->