Musar על כתובות 223:15

The subject of the orbit of the moon, the fixing of leap-months and leap-years, is also related to aspects of the transmigration of souls. The appearance and disappearance of one generation after another reminds one of the lunar cycles. Our sages have jealously guarded what is called the סוד העיבור, the mechanics of how and under what conditions to declare these leap-years and leap-months. [cf. Ketuvot 112, based on Ezekiel 13,9. A collegium of seven scholars would deal with this issue in a closed session: normal sessions were open to the disciples Ed.]