Riddle: When is a first born son totally exempt from Milah?

While there is an exemption for milah for someone whose brothers died due to milah, this is a firstborn son...

Answer: He is the oldest of triplet boys. He was born on Friday bein hashmashos while his two siblings were born on Shabbos proper. His two siblings will have their bris on Shabbos because for them it is certainly the eighth day, however the oldest one's bris needs to be delayed until Sunday as his eight day might have been on Friday (only a brit done on the eighth day breaks Shabbos). Then his two brothers died due to the milah performed on them on Shabbos..which makes the oldest exempt.

Tzvi

[torahskills.org]
[twitter.com]