Thank you for contacting Syncfusion support.
As per the SfCalendar implementation “While setting Calendar FirstDayofWeek as 1 it will display as Sunday” and we have implemented this behavior based on the native JavaCalendar behavior. As per the JavaCalendar the FirstDayofWeek weekdays will return value from 1 to 7, as same calendar will accept values from 1 to 7 and Sunday will be 1 and Saturday will be 7. Kindly set the value as 2 to set Monday as first day of week.
JavaCalendar code snippet for your reference,
Java.Util.Calendar nativeCalendar = Java.Util.Calendar.Instance;
// The value Sunday will set the FirstDayOfWeek as 1
nativeCalendar.FirstDayOfWeek = Java.Util.Calendar.Sunday;
Kindly revert us if you have any concern.
Subburaj Pandian V