Set the first day of the week - you choose
Different countries have different 'first days of the week'. In Europe the first day of the week is considered to be Monday, in the US and Japan it's Sunday, and in some middle eastern countries, it's Saturday. And you can now set the first day of the week in your Gantt chart yourself. Just click the 'settings & plugins' button in the top toolbar and change the 'first day of the week' setting.
The method used to calculate the week numbers of a year depends on what's considered the first day of the week. So changing the first day of the week might also change how your weeks are numbered.
Currently, in 2016, the week numbers in the US and Europe are not in sync. The week that is considered to be week number 5 in Europe is week number 6 in the US.
To add to the confusion, the first day of the week doesn't necessarily need to coincide with the first day of the workweek. In the US, for instance, the first day of the week is Sunday but people start working again after the weekend on Monday.
If you are not sure what to choose, just follow these standards:
Monday: EU and most of the other European countries, most of Asia and Oceania
Saturday: Much of the Middle East
Sunday: Canada, USA, China, Japan, Israel, South Africa, and most of Latin America
When you start working on a new chart, the tool will do the decision making for you. It will make a best guess of the proper timeline setting based on your ip-address.