SUNDAY IS THE "UNIQUE" DAY THAT I CAN RELAX
The word "unique" does exist in English but it can not be used in this context. Spanish speakers like to use it because they think of the word "unico" in Spanish and then bring it across to English. A better word for this sentence would be "only".
So, it should be: "Sunday is the only day that I can relax". If you want to use the word "unique" in English then you need to use it in a context where you are trying to say that something is very original!
For example: "This building is very unique because it uses a lots of glass and metal".
Hope that helps!
Soy el director de Daniel Smith + Partners. Tengo más de 15 años de experiencia enseñando inglés a españoles.