anniversary vs red letter day

anniversary

noun
  • Such a day that commemorates a wedding. 

  • A day that is an exact number of years (to the day) since a given significant event occurred. Often preceded by an ordinal number indicating the number of years. 

  • A day subsequent in time to a given event by some significant period other than a year (especially as prefixed by the amount of time in question). 

red letter day

noun
  • A particularly significant day; a day of personal or sectarian celebration. 

  • A day marked in red on calendars; a church feast day. 

How often have the words anniversary and red letter day occurred in a corpus of books? (source: Google Ngram Viewer )