1)    Some have the custom to fast on the eve of the new month, and they say the prayers of Yom Kippur Katan. The reason that we call the eve of the new month like this is that since on the day of the new month the sins of the entire month are atoned for as we say during the additional prayer of the new month that “it is a time of atonement for all of their descendants”. Even one who does not fast should repent on this day to repair that which he did wrong during the entire month, as the final day of the month is like the final day of the year and then the new month will actually become a “time of atonement for all of their descendants”. (Mishne Berurah 417)

2)    One who wishes to commence fasting on the eves of the new months or to say the special prayers mentioned earlier should specify the first time that he does so that he is only doing it “without a vow” so that if there comes a time when he is unable to fast etc. it will not be considered like breaking a vow. (ibid)