How about "sexta-feira é um dia do semana"?
Can someone again explain for me the purposes and difference between "da", "do", "de"?
Is this correct?
"de" = of (not indicating feminine or masculine)
"da" = "de" + feminine
"do" = "de" + masculine
You're right. The point is that semana is a feminine word, so you use "a". Most of time, words ending in -a are feminine...
"A sexta-feira é um dia da semana." is not right? It's incorrect to use the definite article 'a'?
It is incorrect. For descriptive sentences, don't use the definite article.