英语语法it is the time of the year at which relatives and friends exchange visits 句子成分

it 不是形式主语,就是这句话里的主语,句子主干就是It is the time. 然后用后面的定语从句修饰这里的time,其实应该是两句话：
It is the time. relatives and friends exchange visits at the time.
合并成一句,第一句中的time称为先行词,用关系代词which 来指代time,合并为：
It is the time at which / that relatives and friends exchange visits at. (这里用at是说在这个时间)
at可以放前面,变成at which,而不能为at that,另外也可以把 at which 变为 when,就变成：
It is the time when relatives and friends exchange visits.
希望以上解释能帮助你理解!