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。
区别就是两者意思是不一样，具体的不同如下
on the weekend:是特指某个周末
Whether or not we can catch up on sleep—on the weekend, say—is a hotly debated topic among sleep researchers.
on weekend:是泛指,指周末时间
