it 不是形式主語(yǔ),就是這句話(huà)里的主語(yǔ),句子主干就是It is the time. 然后用后面的定語(yǔ)從句修飾這里的time,其實(shí)應(yīng)該是兩句話(huà)：
It is the time. relatives and friends exchange visits at the time.
合并成一句,第一句中的time稱(chēng)為先行詞,用關(guān)系代詞which 來(lái)指代time,合并為：
It is the time at which / that relatives and friends exchange visits at. (這里用at是說(shuō)在這個(gè)時(shí)間)
at可以放前面,變成at which,而不能為at that,另外也可以把 at which 變?yōu)?when,就變成：
It is the time when relatives and friends exchange visits.
希望以上解釋能幫助你理解!