Yep, I looked it up too (thanks, cementjungle). This is why you should never ask a bachelor questions like that...I was thinking it was gonna be a celebration of yer little nipper growing up to become attracted to same-sex ungulates or cis-normative rubber boots or something.
My thought was that the parents hadn’t told their friends the baby’s sex until after the birth, so this could be like a “come over and see the baby” thing. (However, that would make buying presents other than diapers and wipes difficult.) This is why I thought the 11:30 p.m. hour was strange.