Мій день народження у липні.

Because мій has to agree with the main noun, and the main noun here is день (day), which is masculine singular.

• мій = masculine singular
• моя = feminine singular
• моє = neuter singular

Even though народження is a neuter noun, it is not the head of the phrase. The whole expression is built around день, so мій день народження = my birthday.

Literally, it means day of birth:

• день = day
• народження = birth

Together, день народження is the normal Ukrainian way to say birthday.

So Ukrainian uses a phrase where English uses a single word.

In день народження, народження is in the genitive singular.

It works like of birth, so the structure is literally:

• день = day
• народження = of birth

This is a fixed and very common expression, so learners often just memorize день народження as one unit meaning birthday.

Because in Ukrainian, the present-tense verb to be is usually left out.

So:

• Мій день народження у липні.

literally looks like:

• My birthday in July.

but it naturally means:

• My birthday is in July.

This is completely normal in Ukrainian. You do not usually need a separate word for is in simple present-tense sentences like this.

Because after у/в meaning in, the name of the month usually goes into the locative case.

The dictionary form is:

• липень = July

The locative form is:

• у липні = in July

So the ending changes because of the preposition у/в.

Yes. у липні and в липні both mean in July.

Ukrainian often switches between у and в for euphony—that is, to make the sentence sound smoother. The meaning does not change.

So both are correct, and native speakers choose the one that sounds better in context.

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

The sentence:

• Мій день народження у липні.

is a neutral way to say it.

But you could also say:

• У липні мій день народження.

That puts more emphasis on in July.

Both are grammatical. The difference is mainly about focus and style, not basic meaning.

Yes, absolutely. That is also very natural.

• Мій день народження у липні = My birthday is in July
• У мене день народження у липні = literally At me, birthday is in July, but naturally My birthday is in July / I have my birthday in July

The version with у мене is very common in everyday speech.

Because in Ukrainian, names of months are normally written with a lowercase letter:

• січень
• лютий
• березень
• липень

So липні stays lowercase unless it begins the sentence.

This is different from English, where July is always capitalized.