Agreed.Sunday is always treated as the last day in a week in my area too. As different people have different perpectives about it, it would be more safe by telling the exact date rather than “这个/下个星期日".I'm not sure if there are regional differences regarding this, but where I live, it is true that Sunday is treated as the last day (day 7) in a week.
Edit: According to Wikipedia, Sunday is seen as day 1 in Hong Kong and Taiwan. Can anyone verify this?
It happens in Malaysia too. I am a Malaysian ChineseIn mainland, I have never encounter a person who doesn't treat Sunday as the last day in a week.