Either of these should work, in your page you have it backwards, the cStr needs to go around the Request.Form, not around the switchMonthDay like it is in the file.
The function is manipulating the values as strings only, so the value that is passed to it must be a string also. So if you need to cast the value to a string you should do so around the value itself, then put the function around that. it should be like one of the examples you posted:
<%=switchMonthDay(Request.Form("S_data"))%>
or if you need to caste it as a string use this one
<%=switchMonthDay(cStr(Request.Form("S_data")))% >
Are you getting the same error with this?
