(something astronomical occurring visibly in the sky)
The phrase celestial event in astronomy refers to something
astronomical occurring that is visible in the sky.
The term is often used in calendars or almanacs for predictable viewable
events such as solar eclipses.
Other examples include lunar eclipses, occultations,
conjunctions, oppositions, solstices, phases of the
Moon, and meteor showers.
The term astronomical event often means the same thing,
but celestial event suggest to me a
phenomenon is observable by us in the sky (i.e., celestial sphere),
whereas I can imagine astronomical event may also be meant to
include other events astronomy presumes occur but we can't
directly observe.