"á sumrin" mean more like "summer" in general. For example: Á sumrin fer ég á ströndina = At summer time I go to the beach. You use "á sumrin" to talk more about what is general for the summer season for you. When using "í sumar" it feels like the next thing you will to say has something to do with the summer "this" year. For example: Í sumar ætla ég á ströndina = (this) summer I'm going to the beach. OR Í sumar fór ég á ströndina = (this) summer I went to the beach. "Í sumar" can either refer to a summer season that has passed or has yet to pass that year. Hope this helps! :)