How did we happened?

Prologue

One special night, whilst Sarah cooking a huge dinner with help of her kids.

"Mom?" Ron called her. Sarah turned to her ten year old son and nodded "yes, honey"

"This day every year, you make lots of food which are dad's favorites. What's special about today ?" Ron asked his mom.

"and also both grandpa and grandma come for dinner. What happened today? why is it's so special?" Sana, her seven year old daughter added. Sarah looked proudly at her smart kids and answered "today was the day your dad and me happened". 

There starts her story.......