I should methodically avoid both of your suggestions. With any luck at all, you won't need them.
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English 1b3But if both of my friends are having two separate celebrations, do we say I have friends' thirtieths?Are the separate ones the same or are they having four? - Or is each having two, but two are the same party so they're having three? I suppose it doesn't matter as long as there's more than one, as you've said.
CliveIf you said to me, 'Hello, Clive. Tomorrow I have friends' thirtieths', I would have no idea what you were talking about.