Learn Finnish: Kahdeskymmeneskolmas maaliskuuta on valoisa päivä. - The twenty-third of March is a bright day.

You use the essive case (like kahdentenakymmenentenäkolmantena) when you want to say that something happens on a specific date. However, in this sentence, the date is the subject of the sentence ("The twenty-third... is"). Because it is the subject, it stays in the basic nominative form.

How exactly is the word kahdeskymmeneskolmas built?

It literally translates to "second-tenth-third". In Finnish, when you make a compound number into an ordinal (like 23rd), every single part of the number takes the ordinal form: kahdes (2nd) + kymmenes (10th) + kolmas (3rd).

Why is the month maaliskuu written as maaliskuuta?

It is in the partitive case. When giving dates in Finnish, the month is always in the partitive because you are referring to a specific day out of that month, which is exactly like saying "the 23rd of March" in English.

Is the adjective valoisa related to the word for light?

Yes! It comes from the noun valo, which means "light". The Finnish suffix -isa is often added to nouns to create adjectives that mean "full of" or "having a lot of" something, making valoisa mean "bright" or "full of light".