Piątek pasuje mi najlepiej.

The verb pasować often means to suit, to fit, or to be convenient/appropriate.

In this sentence, pasuje mi literally means is suitable to me / suits me, but in natural English it is often best translated as:

• Friday works best for me
• Friday suits me best
• Friday is the best for me

So Polish uses the verb pasować where English often uses work in scheduling situations.

Mi means to me or for me here.

It is the unstressed dative form of ja (I).
So:

• mi = to me / for me
• mnie = a fuller, more stressed form of me

In Piątek pasuje mi najlepiej, the idea is Friday suits me best or Friday is most convenient for me. The mi shows for whom Friday is suitable.

Both are possible in some contexts, but mi is the normal, neutral choice here.

Compare:

• Piątek pasuje mi najlepiej. = neutral, natural
• Piątek pasuje mnie najlepiej. = incorrect
• Piątek pasuje najlepiej mnie. = possible only in a special emphatic contrast, but not the normal phrasing

In everyday speech, after a verb like pasować, the short dative form mi is what you usually use.

Because piątek is the subject of the sentence.

This sentence is not saying on Friday. It is saying Friday itself is the option that suits you best.

Compare:

• Piątek pasuje mi najlepiej. = Friday works best for me.
• Spotkajmy się w piątek. = Let’s meet on Friday.

So:

• piątek = Friday as the chosen day
• w piątek = on Friday as a time expression

Because najlepiej is an adverb, and it modifies the verb pasuje.

• dobry = good
• lepszy = better
• najlepszy = best

But those are adjectives.

Here we need the adverb forms:

• dobrze = well
• lepiej = better
• najlepiej = best

So:

• pasuje najlepiej = suits best / works best

You are describing how well Friday suits you, not describing Friday as the best noun directly.

Yes. That is also very natural.

Both mean roughly the same thing:

• Piątek pasuje mi najlepiej.
• Piątek najbardziej mi pasuje.

A useful way to feel the difference:

• najlepiej = best
• najbardziej = most

In everyday scheduling, both are common and natural.

No, Polish word order is fairly flexible.

The most neutral version is:

• Piątek pasuje mi najlepiej.

But you may also hear:

• Najlepiej pasuje mi piątek.
• Mi najlepiej pasuje piątek.
• Piątek najlepiej mi pasuje.

The basic meaning stays the same, but the emphasis changes.

For example:

• Piątek at the start highlights the day
• mi earlier can emphasize for me
• najlepiej earlier can emphasize best

For a learner, the original sentence is a very safe and natural pattern.

Yes, in Polish days of the week are normally written with a lowercase letter:

• poniedziałek
• wtorek
• środa
• czwartek
• piątek

In your sentence, Piątek is capitalized because it is the first word of the sentence.

So in the middle of a sentence, you would write:

• Najlepiej pasuje mi piątek.

A rough pronunciation guide is:

• piątek ≈ PYON-tek

A few notes:

• pią- sounds like pyon with a nasal vowel
• ą is a nasal sound; before t, it is often pronounced roughly like on or om depending on context
• stress in Polish usually falls on the second-to-last syllable, so here the stress is on pią

So: PIĄ-tek

It is especially common in scheduling and choosing options, but the pattern is broader.

You can use X pasuje mi najlepiej for many kinds of choices:

• Piątek pasuje mi najlepiej. = Friday works best for me.
• Ta godzina pasuje mi najlepiej. = This time suits me best.
• Ten kolor pasuje mi najlepiej. = This color suits me best.
• Ten termin pasuje mi najlepiej. = This date/time works best for me.

So the structure is very useful beyond just days of the week.

It is in the nominative case.

That is because piątek is the subject — the thing that suits you.

The structure is basically:

• Piątek = subject, nominative
• pasuje = verb
• mi = dative, meaning to me / for me
• najlepiej = adverb, meaning best

So the grammar is:

[subject] + [verb] + [indirect object in dative] + [adverb]

Yes, absolutely. It is a very natural reply.

For example:

• Który dzień ci pasuje? = Which day suits you?
• Piątek pasuje mi najlepiej. = Friday works best for me.

If you want to sound a little fuller or more polite, you could also say:

• Myślę, że piątek pasuje mi najlepiej. = I think Friday works best for me.
• Piątek będzie dla mnie najlepszy. = Friday will be best for me.
• Najbardziej pasuje mi piątek. = Friday suits me best.