weekends为什么要加s at

表示每一个周末，例如：

1、She has to work at weekends from time to time.她偶尔周末还得工作。

2、She doesn't have to work at weekends.她周末不用上班。

3、At weekends I like to sleep late.周末我爱睡懒觉。

4、She gives private English lessons at weekends.她周末给私人教授英语。