celebration vs red letter day

celebration

noun
  • The observance of a holiday or feast day, as by solemnities. 

  • The act, process of showing appreciation, gratitude and/or remembrance, notably as a social event. 

  • The formal performance of a solemn rite, such as Christian sacrament. 

  • A social gathering for entertainment and fun; a party. 

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 celebration and red letter day occurred in a corpus of books? (source: Google Ngram Viewer )