W sobotę dzieci bawią się na placu zabaw przez cały dzień.

Because after w meaning on with days of the week, Polish uses the accusative case.

• sobota = nominative, the basic dictionary form
• w sobotę = on Saturday

This is a very common pattern:

• w poniedziałek = on Monday
• we wtorek = on Tuesday
• w środę = on Wednesday
• w sobotę = on Saturday

So w sobotę is just the correct case form after w in this meaning.

It can mean either, depending on context.

In isolation, W sobotę dzieci bawią się na placu zabaw przez cały dzień usually means:

• On Saturday, the children play at the playground all day, or
• On Saturdays, children play at the playground all day if the sentence is meant more generally.

Polish often does not mark this distinction as clearly as English does. Context tells you whether it is:

• a specific Saturday
• a habitual action

If you want to make the habitual meaning clearer, Polish might use something like W soboty = On Saturdays.

Dzieci means children, so it is plural.

It is the plural form of dziecko = child.

This noun is a bit irregular:

• dziecko = child
• dzieci = children

In this sentence, dzieci is the subject, so it is in the nominative plural.

Because the subject dzieci is plural, the verb must also be plural.

• dzieci = children
• bawią się = they play / they are playing / they amuse themselves

The verb form bawią is the 3rd person plural form of bawić się.

Compare:

• dziecko bawi się = the child is playing
• dzieci bawią się = the children are playing

Bawić się means to play, especially in the sense of children playing, having fun, amusing themselves.

So:

• dzieci bawią się = the children are playing

It is different from some other Polish verbs that English speakers may also translate as play:

• bawić się = play, have fun
• grać = play a game, play an instrument, play in a match

For example:

• Dzieci bawią się w ogrodzie = The children are playing in the garden.
• Dzieci grają w piłkę = The children are playing football.

In bawić się, się is part of the verb. You should learn bawić się as one unit.

Literally, się is related to the reflexive idea oneself, but in many Polish verbs it is simply part of the normal dictionary form and does not need to be translated word-for-word.

So here:

• bawić się = to play
• bawią się = they play / they are playing

It does not usually need an English translation like themselves.

Yes, się often comes after the verb, and that is the most neutral position here:

• Dzieci bawią się na placu zabaw.

But się is a mobile clitic, so word order can sometimes change:

• Dzieci się bawią na placu zabaw.

Both are possible, though bawią się sounds very natural and standard in this sentence.

A useful rule for learners: with many common verbs, placing się right after the verb is a safe and natural choice.

It means at the playground.

Word by word:

• na = on / at
• plac = square / area / yard
• zabaw = of games / of play

Together, plac zabaw is the fixed expression for playground.

After na in this location meaning, Polish uses the locative case:

• plac zabaw = playground
• na placu zabaw = at the playground / on the playground

Because after na when it means in/at/on a place, Polish usually uses the locative case.

So:

• plac = nominative
• na placu = locative

This is very common:

• w domu = in the house
• na stole = on the table
• na placu zabaw = at the playground

So placu is just the locative form required by na in this sentence.

The standard Polish expression is plac zabaw = playground.

Here zabaw is the genitive plural of zabawa = game / play / fun.

This whole phrase is an established compound-like expression, and learners usually just memorize it as one unit:

• plac zabaw = playground
• na placu zabaw = at the playground

So even though the structure may look unusual from an English perspective, it is simply the normal Polish term.

It means for the whole day or all day.

Word by word:

• przez = through / for
• cały = whole / entire
• dzień = day

Together:

• przez cały dzień = all day long

This phrase expresses duration.

Examples:

• Pracuję przez cały dzień. = I work all day.
• Padało przez cały dzień. = It rained all day.

Because przez usually takes the accusative case, and dzień in the accusative singular is still dzień.

So:

• dzień = nominative
• dzień = accusative as well

The adjective must match it:

• cały dzień = the whole day / all day

After przez, that accusative form is exactly what you need:

• przez cały dzień

Polish present tense can cover both meanings, depending on context.

So dzieci bawią się can mean:

• the children are playing (right now), or
• the children play (habitually / generally)

In this sentence, because of w sobotę and przez cały dzień, English might naturally interpret it as either:

• a description of what happens on a given Saturday
• a general Saturday habit

Context would decide which meaning is intended.

Because Polish does not have a separate tense like the English present continuous.

English distinguishes:

• they play
• they are playing

Polish usually uses one present-tense form for both:

• bawią się

So Polish relies more on:

• context
• time expressions
• situation

to show whether something is habitual or happening at the moment.

Yes. Polish word order is more flexible than English word order.

The neutral version here is:

• W sobotę dzieci bawią się na placu zabaw przez cały dzień.

But you could also move parts around for emphasis, for example:

• Dzieci bawią się na placu zabaw przez cały dzień w sobotę.
• Na placu zabaw dzieci bawią się przez cały dzień w sobotę.

These versions may sound less neutral or place emphasis on a different part of the sentence, but they are still understandable.

For learners, the original order is a very natural model to follow.

Yes. A few very useful chunks are worth learning whole:

• w sobotę = on Saturday
• bawić się = to play
• plac zabaw = playground
• na placu zabaw = at the playground
• przez cały dzień = all day / for the whole day

Learning these as fixed expressions will make it much easier to build similar sentences later.