simple past tense and past participle of observe / (of a holiday) Exceptionally celebrated on a date other than the usual date.

present participle of observe

observation

plural of observation

plural of observer

Relating to observation, especially scientific observation.

(degree) To a detectable degree, sufficient to be observed. / (manner) In an observable manner. / (modal) Indicates that the proposition can be verified by observation.

plural of observable

plural of observance

(obsolete) An observer.