>>>So sennight is seven days-but a week is from Sunday to Sunday?<<<

Helen,

I would use week in both of your examples. I would say that something will occur in a week, or a week from today, meaning in 7 days, or next Thursday. I might also say that something will occur next week, meaning sometime in the period that will start Sunday and end the following Saturday. I might also mean the period starting Monday and ending Friday, but in that case it would be obvious that I mean business days from the context.

I agree that most Americans probably know that fortnight means 2 weeks (or they at least understand that it means something between a week and a month), but I can't think of a single time that someone has used fortnight in casual conversation with me.