there is no such a word "先週日" in Japan. by the way, "先週" means last week.

and sentakubiyori is ”洗濯日和” that means the weather is really good, and good for washing clothes(usually sunny day).

洗濯 means laundering,washing(clothes). 日和 means good weather(sunny).

There is no such word as "先週日" in Japan. 
By the way, "先週" means last week.

And sentakubiyori is ”洗濯日和”.
That means the weather is really good, and good for washing clothes  (usually sunny day).

洗濯 means laundering, washing (clothes). 
日和 means good weather  (sunny).