"Today is not Sunday."
What do the characters mean? How to tell what is more formal? How is it decided?
For some just beginning to learn the language, you have to be told what's more formal. :-)
星期 itself means "week," and it is part of the name of each day of the week. (To break it down further, 星 means "star" and 期 means something like "time period.")
日 literally means "sun," also "day".
天 means "heaven," "sky," "day."
Do not forget that 週日 is also Sunday, even though 星期日 and 星期天 are both more commonly used than 週日.