Well, you could use something like
{ IF "{ MERGEFIELD mydate \@YYYYMMDD }" = "{ DATE \@YYYYMMDD }"
"put all your text etc. here" "" }
but unfortunately, that will still produce one blank letter/page/e-mail for
all the records that do not match.
Instead, you could put
{ SKIPIF "{ MERGEFIELD mydate \@YYYYMMDD }" <> "{ DATE \@YYYYMMDD }" }
at the beginning of your merge main document - just be aware that the Word
Help in Word 2000 and later advises you not to use SKIP.