My friend Nanette Tobin and I are eagerly (though maybe quixotically) looking forward to the day when leaving do, meaning “going-away party” enters the American lexicon. In the meantime, we will have to content ourselves with this similar formulation, from the May 2, 2011, New Yorker:
Weddings are a big deal in Great Britain, where “hen parties” and “stag dos” often involve vomiting on the street corners of Magaluf.
notoneoffbritishisms.com 
Love it when I get a mention
I have just discovered this blog. As UK citizen longtime resident it might help me when I forget the derivation of some idiomatic phrases – I can still get odd looks from my American husband of 40+ years. Occasionally it is harder to remember the differences between American English and English English than to speak a third language.
However ‘do’ is not new. It means a good party and can be applied to many occasions. I suspect the usage is originally Northern England although it may have migrated south. Certainly a word in common usage from when I was a child, but with working class links. My snobby mother would never have used it, and to be honest, neither do I.
I can’t give you a citation, but it seems to me that I heard expressions like “That was quite a big do” years ago.
I recall phrases like “quite a big to-do more readily… as for stag and hen nights, well, we can only hope that the custom of the bride to be donning a veil and barreling around on a pub crawl with her mates, singing increasingly drunken karaoke replaces the rather elaborate and expensive bachelorette parties that add to the massive bottom line of friend of the bride.
As Judith says, “do” is a typical expression for a party and has a clear working class tone to it. It is prevalent in South Wales but, in my hearing, more likely to be used by women. Seeing the “Stag Do” looks either ironic or perhaps the use has shifted or varies by region.
“a bit of a do” means a party or function event, sometimes ironically stated i.e. it didn’t go as well as planned.
Definitely no “dos” in Canada, but we do have “stag parties/nights”. Bachelor party is the more formal term I guess.
A good example of the British Stag-Do in this news report (on what was obviously a slow news day) in a local newspaper:
http://www.hertsad.co.uk/news/harpenden_man_dressed_in_bondage_suit_tied_to_lamppost_on_high_street_1_4437835