Use "at the" to describe the place and "in" to talk about specific locations within that place. Example: "The conference will take place at the Cambridge Hotel in the Sterling Ballroom." Reference: https://dictionary.cambridge.org/us/grammar/british-grammar/at-on-and-in-place Regarding places (not time or movement): We use at: - to refer to a position or location which we see as a point: I was sitting at my desk. - to talk about locations at companies, workplaces when we see them as a place of activity: How many people are working at Microsoft? - to refer to activities which involve a group of people: Were you at Lisa’s party? (also at the cinema, at the theatre) - with school/college/university: She always did well at school. - to refer to an address: They once lived at number 12 South George’s Street. - to talk about public places where we get treatments, such as a hairdresser’s or doctor’s surgery: I can’t meet at four. I’ll be at the hairdresser’s until five. - to refer to most shops: Look what I bought at the butcher’s today. We use in: - to talk about locations within a larger area: I know my book is somewhere in this room. Can anyone see it? - to talk about workplaces when we see them as a physical location: She works in an open-plan office. (but we use on when we talk about a farm: I’ve always wanted to work on a farm.) - with class: He found it difficult to concentrate in class.

That's pretty vague. Can you provide an example? There are so many possible variations depending on the event, location, and circumstances.

This would depend on what you wanted to say about the event. If you want to talk about the location or some aspect related to that, then you would say at the event. For example, The event was at the Town Hall and there were a lot of people at the event. On the other hand, if you want to say something about the event itself or some aspect related to the process, then you would say in the event. For example, In the event there were too many long boring speeches.