"Inanahi ko te Rātapu"
Here's a breakdown:
* Inanahi - Yesterday
* ko - is (used for emphasis)
* te - the
* Rātapu - Sunday
This literally translates to "Yesterday was the Sunday."
You can also use a more informal version:
"Inanahi te Rātapu"
This translates to "Yesterday Sunday."
Both options are grammatically correct and convey the same meaning.