The default setting for the first day of the week is Sunday. If you want to change it to e.g. Monday, it can be done in calendar settings.

1. Open Google Calendar

2. Click the gearwheel on the top right corner and choose Settings:

3. Navigate to View options and from there Start week on:

4. Choose Monday from the drop-down menu:

5. Settings are saved automatically. Go back to the calendar and the week will start from Monday