从语法角度讲,yesterday 已为过去时,所以yesterday is satuaday应改为yesterday was satuaday.从句式结构上讲,yesterday is satuaday前面加个连词and或者so就更好了!
以上皆为个人拙见,不知您意下如何?今天是周日，昨天是周六，是客观事实!所以应该是is吧！