У п’ятницю я телефоную сестрі.

Because after у/в to mean on Friday, Ukrainian uses the accusative case.

• п’ятниця = nominative, the basic dictionary form
• у п’ятницю = on Friday

So the ending changes because the noun is part of a time expression, not the subject of the sentence.

In this kind of sentence, у and в usually mean the same thing. Ukrainian often switches between them for easier pronunciation and smoother sound.

So these are both possible:

• У п’ятницю я телефоную сестрі.
• В п’ятницю я телефоную сестрі.

Speakers usually choose the version that sounds more natural next to the surrounding words. У п’ятницю is very common.

Because Ukrainian expresses on Friday with the preposition у/в plus a case ending on the noun:

• у п’ятницю = on Friday

English uses a separate preposition plus an unchanged noun. Ukrainian uses a preposition and changes the noun form.

Because телефонувати takes the person you call in the dative case.

• сестра = nominative, sister as the subject
• сестру = accusative, often a direct object
• сестрі = dative, to the sister / to my sister

Ukrainian treats this verb more like to telephone to someone than English does. So:

• я телефоную сестрі = I call my sister

This is one of the most important things to remember with телефонувати.

Телефоную is the 1st person singular present tense form of телефонувати.

It means:

• I call
• I am calling

depending on context.

Here the ending -ю tells you the subject is я (I).

Grammatically, телефоную is present tense, but with a time expression like у п’ятницю, it can be understood as a planned future action in context.

So depending on the situation, it may mean something like:

• I’m calling my sister on Friday
• I will call my sister on Friday

If you want a more clearly future, one-time completed action, Ukrainian often uses the perfective verb:

• У п’ятницю я зателефоную сестрі. = I’ll call my sister on Friday

Yes, you could, but the meaning shifts a little.

• телефоную = imperfective
  Focuses on the action in a general or neutral way
• зателефоную = perfective
  Focuses on making/completing the call

So:

• У п’ятницю я телефоную сестрі.
  can sound like a plan, routine, or simple statement
• У п’ятницю я зателефоную сестрі.
  clearly means I will call my sister as a completed future action

For a single planned phone call, many learners will often hear зателефоную.

Yes. Ukrainian word order is fairly flexible because the case endings show the grammatical roles.

These are all possible, with slightly different emphasis:

• У п’ятницю я телефоную сестрі.
  Neutral, with time first
• Я телефоную сестрі у п’ятницю.
  More focus on I call my sister
• Сестрі я телефоную у п’ятницю.
  Extra emphasis on my sister

So the original sentence is natural, but not the only possible order.

П’ятницю is pronounced approximately like PYAT-ny-tsyu.

The apostrophe in Ukrainian is not silent punctuation like in English. It shows that the consonant before it is not softened, and the following letter begins a y-like glide.

So п’я sounds roughly like pya, not like a softened пя.

This is why:

• п’ятниця
• п’ятницю

start with something close to pya-.

Usually у п’ятницю means on Friday / this Friday / on a particular Friday, depending on context.

If you want every Friday, Ukrainian normally uses a different expression, for example:

• щоп’ятниці = every Friday
• по п’ятницях = on Fridays

So:

• У п’ятницю я телефоную сестрі. = on Friday / this Friday I call my sister
• Щоп’ятниці я телефоную сестрі. = every Friday I call my sister

Ukrainian often leaves out possessive words like my when the meaning is obvious from context.

So:

• я телефоную сестрі
  can naturally mean I call my sister

If you want to be explicit, you can say:

• я телефоную моїй сестрі

Both are correct, but leaving out моїй is very common when it is already clear whose sister is meant.

Ukrainian does not have articles like a or the.

That means the language does not add separate words for definiteness the way English does. Context usually tells you whether something is:

• a sister
• the sister
• my sister

So сестрі does not need any article at all.