It’s easy to assume that “Summer Of ’69” is about the June, July, and August of 1969. However, Adams clarified what it actually means in an interview with CBS’s The Early Show back in 2008:
A lot of people think it’s about the year, but actually, it’s more about making love in the summertime. It’s using ’69 as a sexual reference.
This explanation adds a whole new layer to the line, “Those were the best days of my life”.