I'm going to the cinema this evening/today evening.
I'm fishing with my granddad on Sunday/at Sunday
Как правильно?

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Both sentences are grammatically correct, but they may have different meanings depending on the context.

"I'm going to the cinema this evening" indicates that you will be going to the cinema later on the current day.

"I'm going to the cinema today evening" is a bit less common, but it still conveys the same meaning that you will be going to the cinema later on the current day.

"I'm fishing with my granddad on Sunday" states that you will be fishing with your granddad on the upcoming Sunday.

"I'm fishing with my granddad at Sunday" is not a common way to express this idea in English. It is more common to use "on" when referring to a specific day.