Esta peça está em promoção hoje.

They are two completely different words:

• esta = this
• está = is (from the verb estar)

The accent mark matters a lot here:

• esta peça = this item / this piece
• está em promoção = is on sale

So even though they look very similar, one is a demonstrative word and the other is a verb.

Because peça is a feminine noun.

In Portuguese, words like this change to match the gender of the noun:

• este = masculine singular
• esta = feminine singular

Since peça is feminine, you say:

• esta peça

If the noun were masculine, you would use este:

• este produto = this product

Peça literally means piece, but in real usage it can mean different things depending on context.

Common meanings include:

• piece / item
• part of something
• a clothing item
• a theater play (peça de teatro)

In this sentence, peça most likely means item or piece of merchandise, especially in a store context. Very often in Brazil, it can refer to a clothing item too.

So here it means something like:

• This item is on sale today.

Portuguese often uses estar for temporary conditions or states.

• ser / é is usually for permanent or defining characteristics
• estar / está is usually for temporary situations, conditions, or current states

Being on sale is seen as temporary, so Portuguese uses estar:

• A peça está em promoção. = The item is on sale.

Using é here would sound unnatural in standard usage.

Em promoção is a very common expression in Brazilian Portuguese. It usually means:

• on sale
• discounted
• being offered at a special price

Word-for-word, it looks like in promotion, but that is not the natural English translation. In this shopping context, the natural meaning is on sale.

Examples:

• Esse produto está em promoção. = That product is on sale.
• Hoje tudo está em promoção. = Today everything is on sale.

Yes. Promoção has more than one meaning.

It can mean:

• a sale / special offer
• a promotion campaign
• a job promotion

For example:

• A loja fez uma promoção. = The store ran a promotion / sale.
• Ele ganhou uma promoção no trabalho. = He got a promotion at work.

In está em promoção, the shopping meaning is the one intended.

Both can appear, but they are not always used in exactly the same way.

• em promoção usually means on sale / discounted
• na promoção often means in the promotion / in the sale event

Compare:

• Esta peça está em promoção. = This item is on sale.
• Esta peça está na promoção de inverno. = This item is in the winter sale.

So for the general idea of an item having a sale price, em promoção is the most standard choice.

Yes, hoje can move around. Portuguese word order is fairly flexible, especially with time expressions.

All of these are possible:

• Esta peça está em promoção hoje.
• Hoje esta peça está em promoção.
• Esta peça hoje está em promoção.

The version with hoje at the end sounds very natural and neutral. Putting hoje first can give a little more emphasis to today.

In everyday spoken Brazilian Portuguese, many people often use essa where traditional grammar would expect esta.

Traditional distinction:

• esta = this, near the speaker
• essa = that, near the listener or previously mentioned

But in casual Brazilian speech, essa peça is often used even when English would say this item.

So:

• Esta peça está em promoção hoje. = correct and clear
• Essa peça está em promoção hoje. = also very common in Brazil, depending on context

Learners should understand both.

Yes. Portuguese words often agree in gender and number.

Here:

• esta is feminine singular
• peça is feminine singular

That agreement is required.

If the noun were plural, it would change:

• estas peças = these items

If it were masculine singular:

• este produto = this product

Normally, in this kind of sentence, you would not say a esta peça unless the grammar specifically requires a preposition plus article-like form.

For a simple subject, you just say:

• Esta peça está em promoção hoje.

In Brazilian Portuguese, demonstratives often appear without an article in this position.

You may also hear combinations like:

• essa peça
• aquela peça

But not usually a esta peça as the plain subject of the sentence.

A rough pronunciation guide is:

EH-sta PEH-sa es-TAH ẽ pro-mo-SAW̃ OW-zhee

A few useful notes:

• peça has an open é sound: PEH-sa
• ç sounds like s
• está has stress on the last syllable: es-TAH
• em often sounds nasal
• promoção ends with a nasal -ão
• hoje in Brazilian Portuguese sounds roughly like OH-zhee

Yes, it sounds natural, especially in written or semi-formal retail language.

It could appear:

• on a website
• in an ad
• in customer service speech
• in a store announcement

Very natural alternatives include:

• Esta peça está em oferta hoje.
• Essa peça está em promoção hoje.
• Hoje esta peça está com desconto.

All of these can work, but your original sentence is perfectly normal.