A forma de "in" e "on" em dias da semana

Há um equívoco. Não está correto. Não se usa "in" com os dias da semana em inglês. A preposição adequada é "on": on Monday, on Tuesday, etc.

In

Use 'in' with months, years and periods of time such as decades or centuries:

In January
In 1978
In the twenties

Use 'in' a period of time in the future to express that an action will occur after that amount of time:

In a few weeks
In a couple of days

At

Use 'at' with an exact time:

At six o'clock
At 10.30
At two p.m.

On

Use 'on' with days of the week:

On Monday
On Fridays

Use 'on' with specific calendar days:

On Christmas day

Bons estudos.

Olá Donay,

E para dizer ''naquele dia'' ?

Ex: On that day I saw you crossing the street = Naquele dia eu te vi atravessando a rua.

Será que o AT também funciona por se tratar de um dia especifico(ambas pessoas envolvidas sabem que dia era) ?

Valeus

Sim, "on that day" está correta. "On" é amigo íntimo (close friend) de "day". Porém, há casos em que você pode omitir "on".

Do Wordreference - USA:

That day I was really sick.
This could mean I became really ill on that day.

On that day, I was really sick.
This could mean that perhaps I had good days and bad days but that that day was especially bad for me.

Bons estudos.