Chasidut do Liczb 30:4

[Any] man who vows a vow [yidor neder] etc (Numbers 30:4) - It appears to me that behold, a person has a piece of God Above, and any person needs to prepare and make a pleasant living space [dirah] for that piece, meaning, in the upper worlds. "When one vows" [yidor] is connected to the expression "living space", "to Ad-nai" indicates the piece of God Above, and this is what the sages said "an unspecified selling of a living space is for a minimum of 30 days" (Arukh HaShulchan, Choshen Mishpat 312:12). This is a hint for a person who would like to acquire a living space for their piece of Above: one needs to fix their actions every 30 days, meaning, direct their thoughts to teshuvah and review [lehader, also praise or embellish] their deeds every 30 days, and this is why the sages have established fasting every erev Rosh Chodesh [day before the beginning of the month] so one could rummage through their deeds, and because of this they hinted that Rabbi Yehoshua ben Levi would review his learning every 30 days. (Brachot 38b)