A Halloween un bambino bussa alla porta in costume.

Yes: in this sentence, a is the preposition used for time, so a Halloween means on Halloween or at Halloween.

Italian often uses a with holidays and special occasions, for example:

• a Natale = at Christmas / on Christmas
• a Pasqua = at Easter
• a Halloween = on Halloween

So this is a normal Italian pattern, even though English uses a different preposition.

Because Halloween is being treated like the name of a holiday. In Italian, names of holidays usually appear without an article in this kind of expression:

• a Natale
• a Pasqua
• a Halloween

So a Halloween is natural, not all’Halloween in standard everyday Italian.

You may occasionally see ad before a vowel, because Italian sometimes adds a d for smoother pronunciation. However, in modern Italian this is much more limited than many learners expect.

Here, a Halloween is the normal and natural choice.

So for learning purposes, use:

• a Halloween

not

• ad Halloween

Because un bambino means a child / a boy, introducing someone not yet identified. It is an indefinite noun phrase.

• un bambino = a child, some child
• il bambino = the child, a specific child already known or already mentioned

So this sentence is presenting the character for the first time: a child knocks on the door in costume.

Literally, bambino is masculine singular, so it can mean boy. But depending on context, it can also be used more generally as child.

If you wanted to be clearly feminine, you would say:

• una bambina = a girl / a little girl

In this sentence, English might translate it as a child if the gender is not important, or a boy if you want to keep the masculine form more explicitly.

Bussa is the third person singular present tense of bussare, which means to knock.

The pattern is:

• io busso = I knock
• tu bussi = you knock
• lui / lei bussa = he / she knocks

Since the subject is un bambino, singular, the verb must also be singular:

• un bambino bussa

Italian often uses the present tense to describe a scene, a typical event, or something happening in a vivid, immediate way.

So un bambino bussa alla porta can sound like:

• a general scene
• a description in a story
• a caption-like statement
• something happening right now

This is very natural in Italian. It does not necessarily mean habitual action only.

Because a + la contracts to alla in Italian.

So:

• a + la porta → alla porta

This kind of contraction is required:

• a + il → al
• a + lo → allo
• a + la → alla
• a + i → ai
• a + gli → agli
• a + le → alle

So bussa alla porta is grammatically normal.

This is just a difference in preposition use between the two languages. Italian says bussare a / alla porta, while English says knock on the door.

So you should learn bussare alla porta as a set expression, not translate the preposition word for word.

In costume means in costume, dressed up, or wearing a costume.

With Halloween, it naturally suggests someone wearing a Halloween costume.

It is a very normal Italian expression:

• essere in costume
• andare in costume
• un bambino in costume

In context, it means the child is costumed, not that there is some special theatrical setting.

Yes, it could go elsewhere. Italian word order is flexible.

The original sentence:

• A Halloween un bambino bussa alla porta in costume.

puts in costume at the end, which sounds natural and lets the sentence unfold step by step: time, subject, action, extra detail.

You could also say:

• A Halloween un bambino in costume bussa alla porta.

This version attaches in costume more directly to un bambino.

Both are grammatical. The original is simply a very natural way to present the scene.