Why seventh day Shabbat is wrong? The answer is simple: Solar Year (365.25 days) and Lunar Year (354 days) ARE NOT divisible by 7. That is it!
If you observe 7th day Shabbat, your calendar drifts by 1.25 days each Solar year or 4 days each Lunar Year. This means, that for example, in 30 years, your calendar will be off by more than a month by Solar calendar!
This is why 7th day Shabbat is totally wrong way to observe Torah Shabbat.