Because 年 ,月 and 星期 are nouns, but 天,日 and 周 can be nouns and measure words. Thus, when we use 天,日,周,it`s unnecessary to add other measure words before them.
I’m a Chinese and I can teach you Chinese for free. Just make me your Wechat friend. My Wechat account is 59684103. Or you can tell me what instant message software you are using?