Isn't an article always needed? So shouldn't this be Η Τρίτη ? Or are there certain times when the article isn't needed?
The article is needed in the course to help you remember the gender of words, it doesn't mean that we always use it. In this case Τρίτη is a perfectly normal response to the question "what day is it today?" (τι μέρα είναι σήμερα;) - and it would be wrong to use an article.