What's the difference between 星期X and 周X?
You have 3 methods when talking about the seven days of the week.


星期[X]

礼拜[X]

周[X]



Here, &quot;X&quot; represents &quot;一，二，三,四，五，六&quot; for &quot;Monday, Tuesday, Wednesday, Thursday, Friday, Saturday&quot;. For case 1 and 2, X could be &quot;日&quot; or &quot;天&quot; for Sunday,for case 3, X should be &quot;日&quot; for Sunday.
What' the difference? 
Well, I think the 星期 and 周 are common to see, while 礼拜 is less but still common(I think it's the personal or regional perference. Thanks for fefe's suggestion). 礼拜 originates from Christianity, and in ordinary life, each type is accepatble.
Why did I use &quot;周三&quot; in that example?
Because I used &quot;本周二&quot; in that example, then why I used &quot;本周二&quot;? When you want to express &quot;this Tuesday&quot;, you could say 


本周二
这个星期二
这周星期二
本周星期二


Each of them is interchangeable. You see, &quot;本周二&quot; is the shortest form, so I used that. When you are talking about the days for a specific week, I think 周 is more common(please note that 周 means week).