* However, the Baal HaMoer (on Rif) explains that the reason that one may not board a boat close to [[Shabbat]] is because it’s likely that there will be life threatening danger which will necessitate a violation of [[Shabbat]]. However, when it is begun earlier there’s no issue because the obligation to prepare not to violate [[Shabbat]] even for life threatening danger doesn’t begin (explained by Mishna Brurah 248:8).
* The Rashbatz 1:21 originates based on the Baal HaMoer’s concept that a convert shouldn’t schedule his Brit Milah on Thursday because it will lead to a violation of [[Shabbat]] (if he doesn’t recover fully) and the same would be true of a baby who had his Brit Milah delayed. This is codified by the Tur and S”A YD 268. [Interestingly, the Taz explains that even if there’s no concern of a violation of [[Shabbat]] there’s an issue of ruining oneg [[Shabbat]] and so it would be forbidden to do the delayed Brit Milah on Thursday or Friday.]
* However, the Shach YD 266:18 argues on the Rashbetz that the Baal HaMoer would permit just like it’s permitted to board a boat on Friday for the purpose of a mitzvah (S”A OC 248:1) and Brit Milah is a tremendous mitzvah. The
Magan Avraham 331:9, Mishna Brurah 331:33, and Sh”t HaRanach 38 (quoted by Tzitz Eliezer 12:43) agree with the Shach.
* The Chida in Birkei Yosef 248 quotes several achronim who defended the Rashbetz by saying that it was only permitted to board a boat on Friday for a mitzvah if one stipulated with the group that they wouldn’t continue to travel on [[Shabbat]] (S”A 248:1) and since there’s no way to make such a stipulation regarding Milah it’s forbidden to do it within 3 days of [[Shabbat]] like the Baal HaMoer.
* Sh”t Yabia Omer YD 5:23, Menuchat Ahava (vol 1, 1:6), and Rav Mordechai Eliyahu (comments to Kitzur Shulchan Aruch 163:4) accept the Chida as halacha.