No, they don't have the same meaning. 'A week before (something) ' means exactly seven days before that event. 'The week before' can refer to any time during the previous week.

I agree with Fiona.

They do not mean exactly the same thing. A week before is more exact - let's pretend that the wedding in question was on a Saturday - the sentence 'he called me a week before' would mean he called you the Saturday before the wedding. The sentence 'he called me the week before' could mean that he called you on any day during that week before the wedding. I hope that makes sense.

I think yes, but is better "the week before". Isn't it? because we are talking in a specific event. I would like to know what think the other partners.