"Today is Saturday."
Translation:ʻO ka Pōʻaono kēia.
Let's hope someone with more knowledge can give a better answer, but in the meantime: He is often "a/an," and you see this a lot with class-inclusion sentences--He kaʻa kēia/This is a car. Aia is a locational word that suggests "there is/there are" either implicitly or explicitly. ʻO can come before a noun (as a nominative marker) and also often begins equational sentences, such as, ʻO koʻu kaʻa kēia/This is my car. Itʻs a grammatical marker lacking direct translation.