Events that pull us from the ordinary enough to instill memories of a moment in time that stay with us for a lifetime.
I remember when President Kennedy was shot. I sat with my seventh grade class our teacher broke the news. It helps me see the classroom, my teacher--things I'd otherwise have forgotten.
On the other hand a small event can also mark a place in time. When my grandson was almost four years old, we stopped at the store. While crossing the parking lot, I stepped in gum. He surveyed my shoe and said, "That happened to me one time at the park." It stunned me that a child that young had a memory of "back when." Stepping in gum marked that moment in time for him. Now, stepping in gum marks that day in the parking lot for me.
Using events big or small to mark time or generate memories in the life of our fictional characters give your readers a "back when" exerpience to which they can relate.
Write of an event marking a moment in time in your life. If you can't think of one, write about the last time you stepped in gum.