By definition (Merriam-Webster dictionary), an Observance is
1- a customary practice, rite, or ceremony
2- a rule governing members of a religious order
3- an act or instance of following a custom, rule, or law
This means that the term is utilized to indicate both religious and non-religious practices. For the sole purpose of straightforward categorization, we have broadly divided Observances in Holy (or Religious) Days and non-religious Observances. Please note that the distinction is often vague and open to subjectivity. We rely on everyone's input to continue improving. So if you think something is "off", please let us know!
-Henry David Thoreau
Holy days or Religious Days may or may not correspond to "Holidays", and observers are typically tied to a religion and may have religious obligations and traditions such as attending Church, gathering with other believers, or reading certain passages of scripture.
These are more typically recognized as days of remembrance, festivities or Holidays not connected to a specific religion.