Last week can mean different things to different people.
I've come across 'last week' being used for a variety of meanings:
- last 7 days until yesterday
- last 7 days until today
- 7 days of Monday to Sunday (inclusive)
- 7 days of Sunday to Saturday (inclusive)
- 8 days of Saturday to Sunday (inclusive)
'Next week' seems to suffer less from this - at least it is used from a fixed day (next Monday, Sunday, or Saturday - depending on when you like to start your week).
One last thing to ponder... If today is Wednesday, then is tomorrow this Thursday or next Thursday or both? Or is next Thursday in eight days?