Assuming that you have a form page and a processing page...Here's a simple example:
Formpage.asp
<%
strDate=request.querystring("strDate"

Missing=request.querystring("Missing"

NotDate=request.querystring("NotDate"

NotToday=request.querystring("NotToday"

if Missing="TRUE" then
errMsg="<font color='#ff0000'>Date Can Not Be Blank. Please Enter a Date!</font>"
ShowMsg=TRUE
END IF
If NotDate="TRUE" then
errMsg="<font color='#ff0000'>" & strDate & " Is Not A Valid Date!</font>"
ShowMsg=TRUE
End IF
If NotToday="TRUE" then
errMsg="<font color='#ff0000'>" & strDate & " Is Not Today's Date!</font>"
ShowMsg=TRUE
End IF
%>
<html><head><title>Form Page</title></head><body>
<form name="myForm" action="processingPage.asp" method="post">
Enter Today's Date
<br>
<input type="text" name="strDate" value="<%=strDate%>">
<%IF ShowMsg then%><%=errMsg%><%end if%>
<br>
<input type="submit" name="btnSubmit" value="Submit">
</form>
</body></html>
ProcessingPage.asp
<%
If request.form("strDate"

<> "" THEN
strDate=request.form("strDate"

ELSE
response.redirect("formpage.asp?Missing=TRUE"

End If
If not isdate(strDate) then
response.redirect("formpage.asp?NotDate=TRUE&strDate=" & strDate)
elseif CDate(strDate) <> Date() then
response.redirect("formpage.asp?NotToday=TRUE&strDate=" & strDate)
End if
' Otherwise everything is OK and you can do your processing
response.write "A-OK!"
%>