If I refer to next week, or last week, do I use ‘le’ or not?

Yes, generally you need the article, as in:

J'ai passé la semaine dernière en vacances.

-- Chris (not a native speaker).

Bonjour Stuart !

Yes you will need to use "la".

Here's the link to our related lesson:
https://progress.lawlessfrench.com/revision/grammar/how-to-express-next-last-with-durations-prochain-dernier

Bonne journée !