Learn Dutch: Mijn hobby is normaal op zaterdag. - My hobby is normally on Saturday.

Yes! Just like we say "on" in English, Dutch uses the preposition op to talk about things happening on a specific day. You will see this with all days: op maandag, op zaterdag, etc.

Why is zaterdag written with a lowercase 'z'?

Unlike in English, days of the week and months of the year are not capitalized in Dutch. You only capitalize zaterdag if it is the very first word of a sentence.

Why doesn't normaal have a special ending to mean "normally"?

In English, we usually add "-ly" to an adjective to make it an adverb (normal -> normally). In Dutch, you just use the base adjective form! So normaal can mean both "normal" and "normally" depending on how it is used in the sentence.

Can I move op zaterdag to the beginning of the sentence?

Yes, you can! But if you start the sentence with a time phrase, you trigger the "verb-second" rule. The verb (is) must stay in the second position, so the subject (mijn hobby) swaps places with it. The sentence becomes: Op zaterdag is mijn hobby normaal.