That depends on how you want the date to appear. To get a merged date to appear like:
5th of June, 2018
for example, you might use a field coded as:
{MERGEFIELD DATE \@ "'{MERGEFIELD DATE \@ d \*Ordinal} of' MMMM, yyyy"}
Note the use of paired double quotes and single quotes.
To see how to do just about everything you might want to do with dates in Word, check out my
Microsoft Word Date Calculation Tutorial - it's in a 'Sticky' thread at the top of the Word forum:
https://www.msofficeforums.com/word/...-tutorial.html. Do read the document's introductory material.