意思不同。1、on weekends通常的周末，每个周末。On weekends he worthy come to see his mother.周末他总是来看他的母亲。
2、on the weekend就指某个周末。I had to work on the weekend because I fell terribly behind.我工作落后太多了，不得不在周末加了班。
on the weekend:是特指某个周末
on weekend:是泛指,指周末时间
the是特指具体的一个,at the weekend和on the weekend是一样的意思,通常用at the weekend
