A servers operating system is chosen in the customer center after the servers provision. This procedure is called "initialisation" and takes between 24 and 48 hours on regular business days.
If a server is initialised Fridays the initialisation often is finished the following monday (in case it is not a holiday).
The initialisation times stated here are standard values and not guaranteed. Generally an initialisation takes place in less than 24 hours even on weekends. In the minority of cases it can take more than 48 hours.