I agree with most of your statement. The only part I disagree with is the "even if it was on their wedding day." You should def be happy for someone but I, personally, don't think it should be announced at someone else's wedding - that actually is their day and it's kind of a jerk, attention-stealing move to announce your engagement at their wedding, unless, of course, you told and asked the couple first and they said it was fine.

Or if it came out on accident like on How I Met Your Mother, with Robin and Ted at Marshall and Lily's wedding.

Depends on the person, I guess. If one of our friends or relatives had announced that on our wedding, my reaction would had been "congratulations, and now we have even more to celebrate!"

Well it would also probably depend on /when/ and /how/ it was announced. Like if immediately after the couple got married, you went and announced, "We're also getting married!!!". It could easily be taken as show stealing, buy also might be taken as "YAY, we'll both be married!!" Depending on how you announced it. But if say, during the reception you told people privately, and not like announce it to everyone, that would probably go over pretty well. Or even during the getting ready stage of the wedding before it occurs, if you announced that "its so wonderful seeing ______ get married! And pretty soon, I'll be doing the same!" People could easily be happy for both parties. But if you went "its so wonderful seeing ______ get married, but good news!!! I'm getting married also!" It sounds a bit more like "but i want i attention to be on me not the ones getting married today."

At a distant relatives wedding (my dad's cousin), two of MY cousins announced they were getting marrI ed. But not to everyone... Only close relatives and they didn't drag it on. And in one of their defenses, the distant relatives wedding was the day after a holiday and my cousin and his fiancee got engaged on said holiday so they were probably still a little overly giddy and stuff. I had a friend who like called everyone he knew right after he got engaged and his voice was like a kids on Christmas, haha. Some people probably just can't hold it in. Also typically, at a certain age, suddenly everyones getting married so it's impossible to avoid stepping on toes a little bit lest you wanna wait 5 years to get married... considering both of my cousins were dating their partners for like 10 years or so before getting married, it's likely the news came out while being harassed about "why have you guys not gotten engaged yet" which yes even happens at people's wedding... and my cousins were probably tired of it after 5 years or so of such questioning... So yeah... "it depends" in essence...

G, Why so Ceer-Ree-Us?

While I do agree that OP shouldn't let the behavior slide, I think going out of their way to be petty about it is a bit much. Being childish and doing something to deliberately be mean to the friend is no better than the friend being an ass in the first place. All it will do is create a bigger issue. Instead of being catty about it, OP needs to talk to her friend and explain why the reaction was in poor taste. If the friend reacts badly again, then it'll probably be time to evaluate if the "friendship" is worth keeping.

...You realize this is three months AFTER the friend's wedding?

Are you sure? I read it as the OP was waiting 3 months after her own engagement to not overshadow the pre-wedding activities and then announced it after the wedding was over (but still on the wedding day).

where did you get that OP announced her engagement on her friends wedding day. It clearly says that OP announced 3 months after the friends wedding stuff was done and over with.. the friend is still pretty much a newlywed friend 3 months later if that is what confused you. Her being as something new, however, should clearly not be the case anymore. OP'S "friend" was/is a total jerkwad. No question about it barring if OP "edited" the story for FML purpose. I try to give OP'S the benefit of a doubt however and just go with it. I could totally see an actual bridezilla being like bent because a whole year should be all about them and their wedding and their newly wedded life and all that bullshit. attention where's suck. I'd have dropped it like it's hot - but on the more burning piece of coal side of the hot spectrum. Ain't nobody got time to deal with that toxic baloney.

Then OP would have had no reason postponing the announcement as they would have gotten engaged three months before the friend's wedding.