名詞 weekday:

1. any day except Sunday (and sometimes except Saturday)

形容詞 weekday:

1. occurring every day of the week except Sunday (and sometimes Saturday)

1. A newspaper published every day or every weekday.
   日報，周報每天或每周出版的報紙
2. The library is open on weekdays only.
   這個圖書館星期一至星期六都開放。
3. I don't stay out late on weekdays.
   工作日的時候我不在外面呆得很晚。

week.day

n.（名詞）
Any day of the week except Sunday, or often except Saturday and Sunday.
周日：除了星期日或常指除了星期六和星期日的一周中的任一天
n.attributive.（定語名詞）
Often used to modify another noun:
周日：常用來修飾其它名詞：
例句：
weekday meetings; a weekday commute.
周日會議；周日上下班交通