El avistamiento fue ayer.

Because avistamiento is a masculine singular noun, so it takes the masculine singular definite article el.

• el avistamiento = the sighting
• un avistamiento = a sighting

In this sentence, el shows that we are talking about a specific sighting, not just any sighting.

Avistamiento means sighting or spotting, especially when something is seen at a distance or noticed for the first time.

It often appears in contexts like:

• avistamiento de ballenas = whale watching / whale sightings
• avistamiento de aves = bird sighting / birdwatching context
• avistamiento de un OVNI = a UFO sighting

It comes from the verb avistar, which means to spot, to sight, or to catch sight of.

Fue is the third person singular of the preterite of ser.

Here, fue means was:

• El avistamiento fue ayer. = The sighting was yesterday.

Spanish uses the preterite here because it refers to a completed event at a specific time in the past.

This is a very common question.

• fue = preterite
• era = imperfect

In El avistamiento fue ayer, the speaker is placing the event at a definite point in time: ayer.

That makes fue the natural choice.

Compare:

• El avistamiento fue ayer. = The sighting happened yesterday.
• El avistamiento era extraño. = The sighting was strange.
  Here era describes what it was like, rather than when it happened.

So:

• fue → completed event / specific time
• era → description, background, ongoing past situation

Because Spanish normally does not use a preposition before ayer.

English says:

• on yesterday ❌
• just yesterday ✅

Spanish works the same way:

• fue ayer = was yesterday

So you simply say ayer, with no en, a, or other preposition.

No. Ayer is flexible.

These are all possible, depending on emphasis:

• El avistamiento fue ayer.
• Ayer fue el avistamiento.
• Fue ayer el avistamiento. (more marked or emphatic)

The most neutral and natural version is the original one:

• El avistamiento fue ayer.

Normally, no, not with this meaning.

El avistamiento fue ayer means the sighting took place yesterday.

If you say era ayer, it would usually sound wrong here, because era does not normally locate a finished event at a specific time like this.

So for a completed event on a specific day, use fue.

A simple Spain Spanish approximation is:

• El avistamiento → el a-bees-ta-MYEN-to
• fue → fwe
• ayer → a-YER

A few helpful points:

• v and b are pronounced very similarly in Spanish.
• mie in avistamiento sounds like mye
• fue is one syllable
• ayer has two syllables: a-yer

If you want to sound more natural in Spain Spanish:

• the y in ayer sounds like a soft consonant, roughly y in yes, though this varies by region.

It is a real and useful word, but it is a bit more specific and sometimes more formal than some basic everyday vocabulary.

You are likely to see it in:

• news reports
• nature and wildlife contexts
• official or written descriptions
• talk about unusual sightings

In casual conversation, people might also use other structures depending on the context, such as:

• vieron algo ayer = they saw something yesterday
• hubo un avistamiento ayer = there was a sighting yesterday

So avistamiento is correct, but it is not the most basic beginner noun.

Yes. That is a very natural alternative.

• El avistamiento fue ayer. = The sighting was yesterday.
  This refers to a known or already mentioned sighting.
• Hubo un avistamiento ayer. = There was a sighting yesterday.
  This introduces the event more generally.

So the difference is mostly about focus:

• el avistamiento → the sighting, already identifiable
• hubo un avistamiento → there was a sighting, introducing it

Because the subject is not a pronoun like él or ella. It is a full noun phrase:

• El avistamiento

Spanish often drops subject pronouns, but not usually the main noun if you need to say what the subject actually is.

For example:

• Fue ayer. = It was yesterday.
  This can work if the context already makes clear what it refers to.
• El avistamiento fue ayer. = The sighting was yesterday.
  This is clearer because the noun is stated explicitly.

Yes. Fue can be from:

• ser = to be
• ir = to go

That is one reason beginners sometimes get confused.

Examples:

• El avistamiento fue ayer. → from ser = was
• Él fue al parque. → from ir = went

You know which one it is from the context.

In your sentence, it must be ser, because the sighting went yesterday does not make sense.

Yes, it is grammatically correct and understandable in Spain Spanish.

That said, whether it sounds fully natural depends on context. It sounds most natural in formal, written, or report-like contexts, for example:

• wildlife reporting
• news
• police or official reports
• descriptions of unusual events

In very casual speech, people might more often say something like:

• El avistamiento fue ayer por la tarde.
• Hubo un avistamiento ayer.
• Lo vieron ayer.

So the sentence is good, but its tone is slightly formal or informational.

Yes, that is a good direct translation.

English also allows:

• The sighting was yesterday.

Depending on context, English might also say:

• The sighting happened yesterday.

That second version sometimes sounds a little more natural in English, but the Spanish sentence itself is perfectly fine with fue.