ob·serve / əbˈzərv/ • v. [tr.] 1. notice or perceive (something) and register it as being significant: young people observe that decisions are made by others. ∎  watch (someone or something) carefully and attentively: Rob stood in the hallway, where he could observe the happenings on the street. ∎  take note of or detect (something) in the course of a scientific study: the behavior observed in groups of chimpanzees. ∎  make a remark or comment: [with direct speech] “It's chilly,” she observed | a stockbroker once observed that dealers live and work in hell. 2. fulfill or comply with (a social, legal, ethical, or religious obligation): a tribunal must observe the principles of natural justice. ∎  (usu. be observed) maintain (silence) in compliance with a rule or custom, or temporarily as a mark of respect: a minute's silence will be observed. ∎  perform or take part in (a rite or ceremony): relations gather to observe the funeral rites. ∎  celebrate or acknowledge (an anniversary): many observed the one-year anniversary of the flood.DERIVATIVES: ob·serv·a·ble adj.ob·serv·a·bly / -blē/ adv.

observe
A. attend to in practice, keep to XIV; celebrate, solemnize XVI;

B. give heed to, watch XIV (rare before XVI);

C. say by way of remark XVII. — (O)F. observer — L. observāre watch, attend to, guard, f. OB- + servāre watch, keep.
So observance XIII (in sense ‘prescribed act or practice’). observant applied to Franciscans of the Strict Observance XV; attentive to rule or law; taking notice XVII. observation †observance XIV; action of observing XVI. — L. observatory (-ORY1) building for making observations. XVII. — F. observatoire.