text: would one say, "i'm going to add a 20 days notice on file", or i'm going to place a 20 day notice on the file. does using "a" merit the change of days to day? even when 20 is plural, does this happen? the "a" has to do with the word "notice", not the 20 days.  accepted usage is "a 20 day notice".  your second choice is correct.