Barney proposing on Ted’s big night

I always wondered why Barney had to propose to robin on THAT night specifically. It was such a big deal for ted. barney and robin should have been there, but instead barney was proposing to robin that night which i’m sure made that night for ted even harder, while already having no love to share it with. i read something a while ago about the reason, but i don’t remember. i know barney wanted ted’s blessing, but why on that night in particular?