When I want to say:
A doctors' conference will be held (at/in) the community center.
By logic, I should use "in" in the above sentence, because I am sure the event is held inside the building, not the outside the building.
However, I wonder why English natives use "at" instead of "in" in the sentence above.
By the way, should I say "center on" or "center at" ?
As an AmE speaker, usually for arenas, stadiums, conference centers, large venues with official names we would use "at", but if the "conference center" is within a larger facility and/or if the reader/listener is familiar with that venue, "in" is fine. I'm not sure what you mean in the second part of your inquiry. Are you refering to dates and times?