Flashback in a Synopsis

My novel takes place a year after important Event A. There's references to it in the first three chapters, and then there's a flashback to it in chapter four to explain it more in depth.

I wasn't sure how to cover that in the synopsis. The logical thing to do seems to be to start the synopsis with a few sentences describing Event A, but if an agent reads the synopsis and then looks at sample pages/chapters, they'll be like, "Where's Event A?"

Should I start the synopsis with "one year earlier," then go into a few sentences about Event A, and then the next paragraph would start with "a year later"?

Carrie in PA said:

What about simplifying it to "One year after the zombie apocalypse,"

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Now I wish I'd just written a zombie apocalypse novel, lol. I would do "One year after Event A," but I feel like I need two or three sentences of explanation/world building. Maybe I'm overthinking it and it doesn't matter if I start with the flashback from chapter 4. Since it's referenced in the other chapters it probably shouldn't seem like it "came out of nowhere."

BenPanced said:

1. events of Chapter One -- mentions/introduces Event A
2. events of Chapter Two
3. events of Chapter Three
4. "In the year before Event A..." -- mentions what led up to Event A

I'm guessing Event A is such a crucial point to your story, so I don't think readers are going to wonder what you're talking about since the characters have carried it through for the first three chapters.

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That makes sense. I will try following your sample list. Thank you!