A family of Microsoft spreadsheet software with tools for analyzing, charting, and communicating data.
Don't you want to use the WEEKDAY function - not the DAY function? The DAY function returns a day of the month from 1 to 31. The WEEKDAY function returns the day of the week, a number of 1 to 7 and when formatted as DDD correctly shows September 1, 2015 as a Tuesday.