missal vs observance

missal

noun
  • A book containing the prayers and responses needed when celebrating the Roman Catholic Mass throughout the year. 

  • A prayer book. 

observance

noun
  • A rule governing a religious order, especially in the Roman Catholic church. 

  • The practice of complying with a law, custom, command or rule. 

  • That which is to be observed. 

  • Observation or the act of watching. 

  • The custom of celebrating a holiday or similar occasion. 

  • Reverence; homage. 

How often have the words missal and observance occurred in a corpus of books? (source: Google Ngram Viewer )