Returns a number in the range 1 through 31, representing the day of the month on which date occurs.
date - any calendar date
Data type returned 
Originated in 
FileMaker Pro 6.0 or earlier
Use Day, for example, to identify the day of the month on which payments are due.
Important  To avoid errors when using dates, always use four-digit years. For more information about how FileMaker Pro handles two-digit dates, see Conversion of dates with two-digit years.
Day(“5/15/2014”) returns 15. This example assumes that the system date format is MM/DD/YYYY.
Day(DateSold) returns the day of the month stored in DateSold.
If(Day(Get(CurrentDate))= 15 and Month(Get(CurrentDate))=3;“Beware the Ides of March”;””) displays the text Beware the Ides of March only when the day of the month returned by Get(CurrentDate) is 15 and the month returned by Get(CurrentDate) is 3; otherwise it displays nothing.
Related topics 
Functions reference (category list)
Functions reference (alphabetical list)
About formulas
About functions
Defining calculation fields
Using operators in formulas