"It is a bad day."
Translation:Tha droch latha ann.
I can answer your first question - The tips for the lesson say that although most adjectives follow nouns in Gaelic, there are exceptions. Droch (bad) and deach (good) are some of the exceptions that precede the noun. As to your second question, I am not as sure, but think I read that one describes (a bad day - any old day) and one defines (a bad day (samhain or anniversary of a death . . .). But I don't fully understand this.
With tha e droch latha, the focus of the sentence is it, you are explicitly describing it as a bad day. I would guess that this construction is most suited to a comparison, such as chan eil e deagh latha, tha e droch latha - it isn't a good day, it is a bad day. I might be wrong, though.