To put it as simple as I can, “dernier” is exactly the same as “last”.

I had an accident last week.
It is the last week of the year.

You can usually change the order too, but it might sometimes not sound right.

The (last 2 / 2 last) weeks have been hard.
Ces (2 dernières / dernières 2) semaines ont ete difficiles

(1), (2), (3) and (5) are perfectly correct.
For the (4), it would rather be “les deux semaines suivantes”

Now, if I had to give you a “rule” and not just answer because “it sound better like that”, it would more difficult…

EDIT:

I just found this link, which seems quite acurate to me, but not perfectly written, let me know if you need help to understand

http://activitesfle.over-blog.com/2015/04/prochain-dernier-place-de-l-adjectif-et-changement-de-sens.html