is "at weekend" or " on weekend". Which one is correct?
Also, I can't tell when shall I say "at weekend" and when to say" at the weekend"

Question

Answer

In American English, we say, 'on the weekend' or 'on weekends'. There is no time where we would say "on weekend".

It is my understanding that 'at the weekend' is British English. Also, in some parts of Britain, they would say 'at weekend'. I'm sure some of the British speakers here can help more with that, but I don't think it's recommended to say it that way.

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