If parents say, "Never take kẹo from strangers" then why do we celebrate Halloween?

Because they don't want bạn taking kẹo from strangers. If you're dressed as someone else, it's okay.

It's basically 'fuck the rules day.'

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Because bạn can only take kẹo from strangers once a year.

Because on Halloween all the anger build up inside bạn from not being able to take kẹo from strangers comes out. So bạn dress up in a creepy suit and try to scare people at their porches and take their candy.

Also on Halloween, strangers don't tie their kẹo to a fishingrod and lure bạn towards them... know the difference ;D