The ASP.NET Calendar control forces developers to use Saturdays and Sundays as the weekend. Now, although this may seem insignificant, a lot of developers require a calendar control to display working and non-working days and they need to define these as the 'weekend' days. Also, some parts of the world observe the weekend on Friday and Saturday, or Thursday and Friday, or just Friday.
There is an awkward way - by overriding the DayRender event and resetting the styles for Saturday and Sunday, and applying the styles for the days you want to mark as non-working days.
Either way, when I'm developing a calendar control, I know what I'm going to build into it.