We'll have/We're having a party next Sunday.

___ a party next Sunday. I hope you can come.
(A) We’re having (B) We’ll
The answer is A. Why is choice B incorrect?

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It is possible but unusual. ‘Will’ future is a simple observation of a future event. ‘Be going to’ future indicates a plan of some kind.
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I’d also say that “will” is used to express decision that has just been made,
while “going to” expresses something that has been planned in details.
A party requires planning.