Weigh in on a stupid argument! From the point of view of today, what year did “last summer” occur in?
2016, since it was last year. For 2017, I would say “this past summer.” It’s this year’s summer, but now it’s over.
2017, since that was the last summer. 2016 would be the summer before last. Er, I think.
Regardless of when it was, I know what you did.
Can you remind me? Because I can’t remember what I did.
The proper answer to the poll is of course 2017.
2017
Last summer means the most recent summer.
Last July I would have said 2016 because the current summer was still active.
Same here. “Last summer” is “last year summer.” I mean, it’s contextual, but I would never say “last summer” to refer to this past summer. Just like, generally in my dialect, next Wednesday means “next week Wednesday.” If it’s a Monday, “next Wednesday” is not the upcoming Wednesday. Similiary, “last Wednesday” is “last week Wednesday.” If it’s Friday and you ask me about what I did “last Wednesday,” it would never occur to me that you are talking about two days ago. The question would be “would did you do on Wednesday?” or “what did you do this past Wednesday” or even “what did you do this Wednesday.” But NOT “What did you do last Wednesday?” But, since this is not a universal understanding, it’s best to disambiguate.
This issue (ambiguous temporal deictics) is well-known by English language teachers, (also see next–next Saturday, etc.)
Typically, clarity is lost when the present time frame isn’t perceived as either close or distant to the time being referred to. So, when speaking in September, most people will understand last summer as the one in the previous year. But in December, speakers often will need to seek and/or offer further clarification.
The most common way is as cochrane mentions, the use of this with ***past ***or coming.
Eg, in a conversation on Tuesday:
*A: The party’s next Saturday.
B: You mean this coming Saturday, the 23rd?
A: No, the one after. The 30th. *
Last summer was 2016. This summer (or this past summer) was 2017. Next summer is 2018. I’m surprised by the poll results so far. (I would have thought 2017 would be unanimous.)
The one in 2016 was “summer of last year”. “Last summer” and “summer of last year” are the same only until “summer of this year” ends.
No, you’ll almost always have disagreement on a question like this.
What year did last month come in? I think you’re going to be inconsistent
You’re aware that your last sentence isn’t even unanimous with your first? 
“Last month” would clearly be November. “Last November” would clearly be (in my dialect) November 2016.
I am large. I contain multitudes. 
If I meant Summer 2016 I would have no issues saying “last Summer”. If I meant 2017 I would say “This past summer” to clarify.
When it’s Friday and someone says “Last Wednesday” you don’t assume they meant two days ago unless you are unfun at parties.
Got it in the first post!
To those of you for whom “last summer” currently means the summer of 2016, I’d like to ask.
What does “last US government fiscal year” mean?
What does “last (U.S.) Congress” mean
In July 2016 what does “last February 29th” mean?
In July 2017 what does “last Christmas break” (from college) mean?
What does “last board meeting” mean? Does it matter if there is only one a year in September or two a year in March and September?
I believe all of those references are unambiguous to those of us who say last summer means the summer of 2017. I suspect they are somewhat ambiguous (or perhaps undefined) to those of you who say last summer means the summer of 2016. Can you give me an algorithm I could use for a computer program to interpret the general phrase “last _____” where _____ refers to a point or period of time?
If you had asked the question in August, the answer would have been 2016. September is where things get confusing and I think they stay that way for October even though October isn’t summer by any stretch of the imagination. But by November, “last summer” is clearly 2017.
As soon as the first day of Fall 2017 occurred, the summer of 2017 became “last summer”. The dictionary definition for this use of “last” is “most recent in time; latest”. As of this writing, the most recent summer occurred in 2017.
That said, I usually phrase it differently if I need to be completely unambiguous.
Do you need the answer Suddenly?