Please note my corrections above. We don't capitalise the first letter of every word in written English. We capitalise the first letter of every sentence, all proper nouns and the personal pronoun "I". Also note that I have changed your thread title so that it is unique and relevant to the thread.
You might not get a straight answer to this question! On Thursday, I would call the coming Sunday "this Sunday" and the Sunday after that "Sunday week". However, if someone said "next Sunday" on a Thursday, I would initially assume they were referring to the day three days later, but I might ask them to clarify.
if someone said "next Sunday" on a Thursday, I would initially assume they were referring to the day three days later, but I might ask them to clarify.
This is a true story that happened to me last Wednesday. I was talking to two people—an intermediate learner and another native speaker. The learner said "next Saturday" (meaning in three days' time) and both I and the other native speaker understood that he was referring to the day in ten days' time. I think the vast majority of native speakers would have misinterpreted what he said in the same way as we did.
As emsr2d2 says, if the day is coming up, we normally say this Sunday.
On a Thursday, I would interpret 'next Sunday' to be ten days away. If referring to the Sunday three days away, I would use (and expect to hear) 'this Sunday', 'this coming Sunday', or perhaps even simply 'on Sunday'.