at weekends表示某个具体的周末；on weekends表示特定的的某个周末（比较正式用法）。在周末（美式英语）：on the weekend、on weekends；在周末（英式英语）：at the weekend、at weekends。
on weekends
1、Jim gardened at the homes of friends on weekends.
吉姆周末帮忙打理朋友家的花园。
2、Bill would visit on weekends.
周末比尔会来。
3、They knock around on weekends in grubby sweaters and pants
他们周末穿着邋遢的毛衫和裤子到处闲逛。
at weekends
1、At weekends the roads are jammed with holidaymakers coming to gawp at the parade
周末的时候，公路上便挤满了前来观看游行的度假游客。
2、I don't do the housework at weekends. I play tennis with my friends.
周末我不做家务。我和朋友打网球。
3、At weekends, we can go to the outskirts for a picnic with our friends.
在周末，我们能为和我们的朋友一次野餐去近郊。