Learn Portuguese (Portugal): Eles vão para Londres nas férias.

Both a and para can mean "to", but they express different lengths of time. You use a for a quick trip where you will return soon (like going to the supermarket or the beach for the afternoon). You use para when traveling for a longer stay, or moving somewhere permanently. Since they are going for their vacation, they will be staying a while, making para the correct choice.

Why is "for the holidays" translated as nas férias? Shouldn't it use a word like para or por?

In English, we say we go somewhere "for" the holidays, but Portuguese phrases this differently. It uses the preposition em (in/on/at) to talk about the time period. Nas is a mandatory contraction of em + as (the feminine plural word for "the"). So, literally, they are going to London "in the" holidays.

Is férias always plural?

Yes! The word férias (vacation / holidays) is always plural in Portuguese, even if you are just taking one single vacation. Because it is a feminine plural word, it forces the words connected to it to match, which is why we must use the plural nas instead of the singular na.

Why is there no "the" before Londres? E.g., para a Londres?