Breakdown of Dziś jest piękny dzień.
być
to be
piękny
beautiful
dzień
day
dziś
today (short form of dzisiaj)
Questions & Answers about Dziś jest piękny dzień.
Why is piękny dzień in the nominative case here? Doesn't the verb jest usually take the instrumental case?
You use the instrumental case when you connect two nouns with the verb być (to be), like in On jest nauczycielem (He is a teacher). However, dziś is an adverb, not a noun. Because we aren't saying 'Noun is Noun', the phrase piękny dzień is actually the subject of the sentence and stays in the nominative case. The sentence structure is literally 'A beautiful day is (happening) today.'
Is there a difference between dziś and dzisiaj?
No, they mean exactly the same thing and you can use them interchangeably. Dziś is just a slightly shorter version of dzisiaj. You will hear both used very often in everyday spoken Polish.
How would this sentence change if I wanted to talk about a beautiful night (noc)?
Adjectives must always match the gender of the noun they describe. Dzień is a masculine noun, so it takes the masculine adjective ending -y (piękny). Because noc is a feminine noun, you would use the feminine ending -a, making the sentence: Dziś jest piękna noc.