In Fisher’s theory, the timing of income is irrelevant: Consumer can borrow and lend across periods.
Example: If consumer learns that her future income will increase, she can spread the extra consumption over both periods by borrowing in the current period.
However, if consumer faces borrowing constraints (aka “liquidity constraints”), then she may not be able to increase current consumption
...and her consumption may behave as in the Keynesian theory even though she is rational & forward-looking.