I have never been a JW, nor any of my family, but I knew about 1975 because when I was a teenager, my parents had a young JW employee who came to work for the family business in the late 1960s.
It was a plant nursery and the employee explained he would need to know how to grow things in the paradise that would exist when the current world order ended.
In December 1974 he handed in his notice, telling my parents that he was sad that he would not see them again and begged them to come to the KH, but they were not the least bit interested or worried. He was very socially awkward and never spoke to my sister or me and kept his eyes lowered if we passed by or brought him a cup of tea.
I assume he did full time door knocking during 1975. His parents sold their large house and donated the money to the Watchtower and all the large family moved into a caravan.
Needless to say, in January 1976, he came to ask for his job back, but he rarely spoke about his religion after that.
I recall my parents laughing with their friends at how crazy his ideas were, and although I had no more brushes with JWs until 2008 (when a young friend was sucked in so I joined this site for advice), I never forgot the date of 1975.