Add simple spec of the 'Indicate availability' feature

Supplementing my comment in #2, here's a very simple example of what such specifications could look like for ARNOLD.
Y2K started on a Saturday, making the first week of that year January 3rd through 9th. That is, assuming you're week starts on Monday. If it doesn't, we might get into some more fundamental discussions here.
Of course, I still understand the scenario, but an automated test derived from this might not agree with your use of `next week`. Simplest solution is to up the year by 1; 2001-01-01 was a Monday. :) Still though, shouldn't `the next week` then be called `this week`?