The first day of the week is controlled by the chosen LOCALE_ID. For instance, if the locale is set up to 'en-GB', then the component will display the week from Monday. Here is a simple plunker demo that demonstrates the required configuration:
Basically, the required culture should be loaded:
and then the correct LOCALE_ID provider should be defined:
// Set default locale to bg-BG
provide: LOCALE_ID, useValue: 'en-GB'
The component will use the defined LOCALE_ID and will reflect the firstDay configuration as expected.