No dia 15: In x at x on the (15th)

Avatar do usuário Lorena Batista 60 2
Ola pessoal! Na frase: A prova será no dia 15; uso a preposicao in, or ou at? Obrigada
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Avatar do usuário PPAULO 45365 6 35 800
None of the above. You would use "on".

The exam will be on Monday.
The exam will take place on Monday.
The exam takes place on Monday. (more informal).

The exam will be on Monday, 25th. (The exam will be on Monday, the 25th. - by "academic writing" standards )

The exam will be on Monday, 25th June at 9:30 h.
The exam will be held on Monday 25th.
The exam will be on Monday, June 25th 2018, between 15:30 and 17:00 (90 minutes)

The exam will be held next Monday.
The exam takes place next Monday.
The exam will take place on June 25th. (here meaning that the other person knows that it falls on Monday) .