I think the "prank" is that now whenever someone cites that day he knows it's the wrong day. I find it kind of funny the cult of Mac has that day wrong.
I once met an Englishman, who assured me he was from South Africa, after talking with him for like 10 minutes, he starts cracking up... when I asked why he said "Because I'm actually English!!"
This is a cultural thing. If you must spend time with British people (or some of the ex-colonies) you need to learn how to recognise (and protect yourself from) sarcasm and "jokes" (what you might see as lies and cruelty).
When I was 21 in the US we played a mean game of seeing what we could make Americans believe - and we came up with some truly bizarre stuff.
I think that English people do this as a proxy test of intelligence - it is a continuous competition. Maybe an English person can correct me?
