I was calm for the 6 weeks between submission and the viva too. I think my 'que sera sera' attitude helped a lot. I viewed the big task as having submitted the thesis, and I knew that the examiners' decision would be largely based on that. So I'd done the hard work already. The viva can swing things, especially in marginal cases, but it's not all down to that final day.
So I relaxed. I prepared - as I've explained here a lot - in my minimal (very minimal!) way. And I wasn't at all nervous, until about 1 hour before the viva started, when I was sitting having a hot chocolate, and looking at other people around me, and feeling extremely unreal ...