celebri

One root, no seam. Celebri starts with the same three letters as celo, a goal or target, and there the resemblance ends. There is no -ebr- suffix in Esperanto and no way to cut this word: celebr- is a single international root taken whole from Latin. Aiming at something is celi; honouring something is celebri. The two never meet.

It is the solemn one. English uses celebrate for a birthday party and for a requiem mass alike, and Esperanto splits the job. Festi is what you do with cake, music and friends. Celebri is what a priest does at an altar, what a state does at a monument, and what a poet does to a subject worth praising: it implies ceremony, ritual and a certain gravity. La pastro celebras la meson. La familio festas la naskiĝtagon. Getting these the wrong way round is a harmless but very audible slip.

The stretched sense. Celebri also means to extol, to sing the praises of, and that use is common in older Esperanto prose: la poeto celebras la liberecon. It is close to laŭdi, to praise, but grander and more public. Fama and glora sit nearby as adjectives.

Round it out. A ceremonio is the event, a rito the fixed form it follows, a festo the celebration in the ordinary sense, and the calendar article covers which Esperanto festivals a learner is likely to be invited to.