Omar, you could use addDays([%BeginOfCurrentWeek%],-1) to extract 1 day from Monday to achieve what you want. It's not what you really want, but it works.
It would be better to do it just with the BeginOfCurrentWeek token but that doesn't work (BUG?). If that one isn't working, you could file a bug report.
Edit after comment Rom van Arendonk:
addDays cannot be used. Used [%BeginOfCurrentWeek%] - [%DayLength%] instead.
I just had a look, at the moment this first day of week is always Monday.
There are several ways this could be improved. My initial thought is that it should be dependent on the timezone that you set for a certain user. This means that individual users can have different first days of the week. The calendar picker ideally would use this setting too.
However this might be inconvenient if you really count on the first day always being Sunday or Monday. I'm not sure what cases there are for this. For example, what are you using the begin of current week for in this xpath query?