a browser a scris:No, it's not! And I'm so sure about it...Look:
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on = preposition used to show the date or day when something happens: He's due to arrive on 14 February. I'm working on my birthday.
There are many other situations when on is used...but this one is related to our case.
at = preposition used to show the place or position of something or someone: She's at the library. Used to show the time: The miting starts at three.
...from Cambridge Learner's Dictionary (University Press - 2001.
All I can say is that this is how the English speak. We would say:
What are you doing on Monday?
What are you doing on Ann's birthday?
....but when it comes to talking about the weekend it has always been:
What are you doing at the weekend?
...and just to confuse the matter further, when talking about months we say:
What are you doing in July?
I'm not an English teacher so cant say why, all i know is this is how English is spoken! My south african wife says both sound wrong and she would say 'what are you doing for the weekend?'
You mentioned I make 'mistakes' as in the plural. What else have i said that doesnt make sense? 