A castração foi ontem.

Because castração is being used as a specific event: the castration.

In Portuguese, it is very common to use the definite article with nouns in places where English might sometimes omit it. Here, a castração refers to a particular procedure already known from the context.

Also, castração is a feminine noun, so the article is a, not o.

A very useful clue is the ending -ção. Nouns ending in -ção are almost always feminine in Portuguese.

Examples:

• a castração
• a informação
• a situação
• a estação

So a castração is the natural form.

Here, foi means was.

It is the preterite form of ser for ele/ela/você:

• foi = was

So:

• A castração foi ontem. = The castration was yesterday.

Important: foi can also mean went, because it is also a form of ir. Portuguese has this overlap:

• Ele foi ao mercado. = He went to the market.
• A festa foi ontem. = The party was yesterday.

In your sentence, it clearly means was, not went.

Because the sentence refers to a completed event at a specific time: ontem.

Portuguese normally uses the preterite (foi) for something that happened and is finished:

• A castração foi ontem.

Using era would usually sound wrong here if you mean the event actually took place yesterday. Era is more for background, descriptions, repeated past situations, or sometimes scheduled/expected timing in certain contexts.

So for a one-time completed event, foi is the normal choice.

Because ontem already means yesterday by itself, and it normally does not need a preposition.

Examples:

• Foi ontem. = It was yesterday.
• Cheguei ontem. = I arrived yesterday.
• Ela saiu ontem. = She left yesterday.

This is similar to English, where you also usually just say yesterday, not on yesterday.

Yes. Portuguese allows some flexibility with word order.

These are all possible:

• A castração foi ontem.
• Ontem foi a castração.
• A castração ontem foi... — possible in special contexts, but less neutral

The most neutral version here is:

• A castração foi ontem.

If you start with ontem, you give more emphasis to the time:

• Ontem foi a castração. = Yesterday was the castration.

Yes, it is natural, especially in context.

It sounds like a brief, practical statement, for example about a pet:

• A castração foi ontem.
• Ele ainda está se recuperando.

In everyday Brazilian Portuguese, this kind of short sentence is very common.

If someone wanted to be more explicit, they could also say:

• A castração foi feita ontem. = The castration was done yesterday.
• O procedimento foi ontem. = The procedure was yesterday.

But your sentence is completely normal.

It can. Both are possible, but they are slightly different in style.

• A castração foi ontem. = very direct, simple, common
• A castração aconteceu ontem. = more explicitly happened yesterday

Portuguese often uses ser + time expression with events:

• A reunião foi ontem.
• O jogo foi sábado.
• A prova foi de manhã.

So foi ontem is a very standard pattern.

In everyday Brazilian Portuguese, very often yes.

If someone says:

• A castração foi ontem.

many listeners will naturally think of a pet or another animal, especially in common daily conversation.

The word can also be used in broader or more technical contexts, but in ordinary speech in Brazil, it is strongly associated with veterinary situations.

Possibly, depending on context, but the meaning is not always exactly the same.

In veterinary and everyday use in Brazil, castração is extremely common for neutering/spaying animals. It is the word people most often use in casual conversation.

Esterilização can sound more technical and may focus more specifically on sterilization. In many real-life situations, Brazilians would still prefer castração when talking about pets.

So if you are talking naturally about a dog or cat, castração is usually the most idiomatic choice.

In Brazilian Portuguese, it is approximately:

cas-tra-SÃO

More carefully:

• cas = like cahs
• tra = light trah
• ção = nasal são

A rough English approximation is:

• kahs-trah-SOWN
  but with a nasal ending, not a full English own

The stress is on the last syllable:

• cas-tra-ÇÃO

Yes, very common.

Portuguese frequently uses ser to locate an event in time:

• A reunião foi ontem. = The meeting was yesterday.
• O show foi no sábado. = The concert was on Saturday.
• A aula foi de manhã. = The class was in the morning.

So A castração foi ontem follows a very useful and common structure.

Yes, if the context is clear.

For example:

• Quando foi a castração?
• Foi ontem.

Portuguese often leaves out information that is already understood from the conversation.

So:

• A castração foi ontem. = full version
• Foi ontem. = shorter version, if everyone already knows what it refers to