What is the difference between 星期日 and 周日?
周日 is formal(on TV news) and oral, 星期天 is mostly oral. 星期日 is what Cantonese use orally(Hongkong, Canton), mandarin hardly.
We also use 礼拜天(mandarin)礼拜日(Cantonese) orally a lot.
Most possible combination of is above. btw There is no 周天.