Learn Italian: La domenica è il settimo giorno della settimana.

The Italian word for seven is sette. For ordinal numbers (first to tenth), you drop the final vowel of the normal number and add -imo. So sette becomes settim- + -o = settimo. Because it describes giorno (a masculine word), it keeps the -o ending.

Why is it la domenica (feminine) but il settimo giorno (masculine)?

Domenica is the only day of the week in Italian that is feminine, which is why it uses la. All other days (like lunedì and martedì) are masculine. However, the word giorno (day) is masculine, so it takes the masculine article il and forces the adjective settimo to be masculine too.

Why do we need La before domenica? Doesn't that mean "The Sunday"?

Yes, it literally means "The Sunday". In Italian, when you talk about a day of the week in a general sense—like stating a fact about Sundays or saying you do something every Sunday—you use the definite article (il or la). If you were just talking about an upcoming Sunday, you would leave it out (for example, a domenica for "see you on Sunday").

How is della formed in della settimana?