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!