So... isn't it just safe to ALWAYS say "La dernière fois"???

Hi Francesca,

Technically speaking you would put 'dernier' before the noun when it is part of a series -

Venez chez nous le dernier week-end de juillet = Come to us on the last week-end in July

Le dernier jour de la semaine est le dimanche = The last day of the week is Sunday

But if it is placed after the noun, it means the one just gone -

Le week-end dernier, nous sommes allés à Paris = Last week-end, we went to Paris

Dimanche dernier nous avons fait une belle promenade en forêt = Last Sunday, we had a lovely walk in the woods

Hope this helps!

Other people might be using la fois dernière and it's always good to understand the bigger picture, I guess. I've heard it used when someone wanted to stress particularly the fact that she was referring to the last of a sequence of instances.