В воскресенье этот магазин не работает.

Here в is used in a time expression meaning on, and воскресенье is in the accusative.

The tricky part is that воскресенье is an inanimate neuter singular noun, so its accusative form looks the same as its nominative form. That is why you do not see any change.

Compare:

• в среду = on Wednesday
• в пятницу = on Friday
• в воскресенье = on Sunday

So the case is changing, but with воскресенье the form just happens to stay the same.

In Russian, days of the week are normally written with lowercase letters.

So:

• понедельник
• вторник
• воскресенье

This is different from English, where Sunday and Monday are capitalized. In Russian, you only capitalize them if they begin a sentence or are part of a title.

Because магазин is a masculine singular noun, the demonstrative этот has to agree with it.

Russian words like this change form depending on gender, number, and case:

• этот = masculine
• эта = feminine
• это = neuter
• эти = plural

Since магазин is masculine, the correct form is этот магазин.

Russian does not have articles, so there is no direct equivalent of English the or a/an.

Whether something is definite or indefinite usually comes from:

• context
• word order
• words like этот = this

So этот магазин literally means this store, not just the store. But in many contexts it also makes the noun sound clearly specific, which can feel similar to English the.

Yes, работает literally means works / is working, but in Russian it is very commonly used for:

• shops
• offices
• websites
• machines
• services

So магазин работает means the store is open / operating, and магазин не работает means it is not open / not operating.

This is very natural Russian. English usually says the store is open/closed, but Russian often uses работать / не работать instead.

Работает is the 3rd person singular present tense form of работать.

It is singular because the subject is этот магазин, which is singular.

Compare:

• Этот магазин не работает. = This store is not open.
• Эти магазины не работают. = These stores are not open.

So the ending changes to match the subject.

It can mean either, depending on context.

Very often, a sentence like В воскресенье этот магазин не работает is understood as a schedule/habitual statement, so in natural English it may be translated as This store is closed on Sundays.

But in the right context, it can also mean This store is not open on Sunday referring to one specific Sunday.

Russian often leaves this kind of distinction to context.

Both can refer to Sunday, but they are used a little differently.

• в воскресенье = on Sunday
  This can mean one Sunday or a general Sunday situation, depending on context.
• по воскресеньям = on Sundays / every Sunday
  This form more clearly emphasizes a repeated, habitual action.

So if you want to make the recurring meaning very explicit, по воскресеньям is often better.

For example:

• Этот магазин не работает по воскресеньям. = This store is closed on Sundays.

Yes, Russian word order is fairly flexible.

This version starts with В воскресенье to set the time first, almost like saying As for Sunday...

Other possible orders include:

• Этот магазин не работает в воскресенье.
• Этот магазин в воскресенье не работает.

All of these are grammatical. The difference is mainly one of focus and emphasis, not basic meaning.

The original sentence sounds very natural if the speaker wants to foreground the time expression first.