The Is day equals operator is used to select dates with a day value that exactly matches the DD component of the date that is entered in the condition input field. All other date components are ignored.
Setting the Date condition of a Birth date field to Is day equals, and entering 12/12/1978 as the input field value, selects all those contacts whose birth day matches that value. In this instance, all contacts born on the 12th of any month are selected.
The Is day equals operator applies to date data types.
See Input field types for more about each input field.