1.you should say"今年8月”.It means August.2016. 2.If you say "去年8月”.it refer to the August of last year.So it's August.2015. 3.The sentence of"上个8月”means the August previous. It also refer to August.2015(In fact .native Chinese speakers don't say like this.And I seldom use it) 4.If u just wanna express last summer "去年夏天"is the correct answer.but it isn't the August.2016.(because the year of 2016 is not over.So only "今年夏天" can refer to August.2016) At last the word "夏天" have a same mechanism with "8月”. Just two different words. I'm a Chinese Student. Hope it helps.

1.今年八月or just八月, the contex will tell us, if it's talking about the past. 2.去年夏天

Dio, Thanks a lot for your explanation! I cannot like it because it is a comment, but I really love it, as it is very teaching. I wish I will be able to read Chinese Poetry some day. :)

Some supplements.Referring to the time, the single word “今”, "去", "上个" has different meaning. "今" means "this one"(the one I'm experiencing). When the unit is day, like 今天, it means "today"; when the unit is year, like 今年, it means "this year". "去" means "the one has just passed". But you can't combine 今 with other time-unit freely, I recommend you regarding "去年" as a proper word. "上个" means "the last one". For example “上个八月”, the timing unit is month, and each year has only one August. So in contrast to August in this year, "the last one" refers to last year's August. In Chinese, "今" and "去" usually appear together to express the concept of "contemporary" and "old time". In this way, they are two opposite words. If you have any knowledge of Chinese poem(like the Tang Dynasty's or the Song Dynasty's), you will find this usage was very common in old Chinese literature. (It's still common nowadays. People will use it when they want to express in a literate way).