Temporal u + locative

You already say on Wednesday as u sredu and on Friday as u petak — the preposition u followed by the accusative. Serbian uses the same little u to say in June or this month, but there the noun takes a different ending: u junu, u ovom mesecu. That ending is the one you know from u gradu and u školi — the locative.

A day is a point, a month is a space

Think of it this way: a day of the week is a single point you land on, so it takes the accusative. A month, a year or any longer stretch of time is a space that things happen inside, like a city or a room — so it takes the locative, the case of being in somewhere.

Both can sit in one sentence: U sredu imam ispit, a u julu idem na odmor. — On Wednesday I have an exam, and in July I go on holiday.

The twelve months after u

All the months are masculine and end in a consonant, so they all take the masculine locative ending -u. The four months in -bar drop the a of -bar when the ending is added: septembar → u septembru, oktobar → u oktobru. Januar and februar keep theirs: u januaru, u februaru.

A period with a word in front

When month or year has a word in front of it — this, last, next, which, new — that word goes into the locative too, with the endings you already know from u ovom gradu and u novoj kući: -om / -em before masculine mesec, -oj before feminine godina.

U kom mesecu? is the question for all of this: U kom mesecu je tvoj rođendan? — U septembru. You will also hear the longer u kojem mesecu; both are correct.

Do not put a month in the accusative just because a day goes there: it is u junu, never u jun; u septembru, never u septembar. Ask yourself: a day of the week? Accusative. A month or a year? Locative.

You already know the bare genitive ovog meseca, prošle godine (no preposition). It means the same as u ovom mesecu, u prošloj godini and is just as correct — the version with u stresses inside, during that period. What you cannot do is mix them: u prošle godine is wrong.