Days of the Week:

No Domingo - On Sunday
Na Segunda-feira - On Monday
Na Terça-feira - On  Tuesday
Na Quarta-feira - On Wednesday
Na Quinta-feira - On Thursday
Na Sexta-feira - On Friday
No Sabado - On Saturday

Note that every weekday ( segunda to sexta ) is feminine, but sábado and domingo are masculine. Also note that you often talk about weekdays without "feira". For example, "Vou sair de férias na segunda, or I am going on vacation on Monday".
Ref.: Beginner's Brazilian Portuguese