Try the following:
a. use Alt-F9 to display the merge field, which will probably look
something like

{ MERGEFIELD mydate }

or

{ MERGEFIELD mydate \*Mergeformat }

Type a "date format switch" after "mydate " so the field looks like

{ MERGEFIELD mydate \@"MMMM D, YYYY" }

or

{ MERGEFIELD mydate  \@"MMMM D, YYYY" \*Mergeformat }

Select the field and press F9 to "execute" it, then press Alt-F9 and try the
merge again.

Peter Jamieson