The Jekkish practice in marCheshvon is to observe BeHaB the last 2 Mondays in MarCheshvon with the intervening Thursday, so “the plumber” what I said was correct.
1) Because as days get shorter, it makes fasting a shorter amount of time.
2) The Mabul commenced marCheshvon 17th, and observing last 2 Mondays in marCheshvon, makes marCheshvon 17th, the earliest possible date that Sheni Kamo can come out on.
3) It simulates Mishna in Taanis when 1st Taanis in case rain did not fall was be marCheshvon 17th.