Learn Finnish: Onko kolmaskymmenesensimmäinen joulukuuta aina kylmä? - Is the thirty-first of December always cold?

It is made by attaching three ordinal numbers together into one long word: kolmas (third) + kymmenes (tenth) + ensimmäinen (first). Unlike English where only the last part is an ordinal ("thirty-first"), Finnish makes every single part of the number an ordinal!

The current lesson is about using the essive case for dates. Why is kolmaskymmenesensimmäinen in the basic nominative form here?

We only use the essive case when we want to say that something happens on a certain date (like "I will travel on the 31st"). In this sentence, the date itself is the subject—we are simply asking what the 31st is like. Therefore, it stays in the basic nominative form.

Why is the month joulukuu written in the partitive case as joulukuuta?

Whenever you express a date in Finnish, the name of the month must be in the partitive case. You can think of it as literally saying "the thirty-first [day] of December".

Just as a refresher, why does the sentence start with Onko instead of just on?

To turn a regular statement into a yes/no question in Finnish, you move the main verb to the front of the sentence and add the question ending -ko or -kö. So on (is) becomes onko (is it?).