the.com/observance

means The act of keeping a custom, rule, law, or ritual — doing what tradition, duty, or holiday calls for.

from From Latin observare — 'to watch, guard, pay attention to' (ob- 'toward' + servare 'to keep, preserve'). The same root gives us 'observe' in the sense of watching, which is why the word carries a double sense: to observe a holiday is to watch over it faithfully, the way a sentry watches a post — attention turned into duty.