Is this sentence correct?
She walks in Monday instead of Friday .

Question

Answer

It needs context, but in very informal American English, the meaning could be that she was expected on Friday, but actually came on Monday.

Answer

Hi Navegante,
If you mean to say that she walks every Monday, the sentence can be:

She walks ON Mondays instead of Fridays.

If you mean a particular Monday in the future, the sentence can be:

She will walk ON Monday instead of Friday. 
 
Cheers,
Lance