If you can't remember the first day of preschool or kindergarten, you can always describe the first day of high school or the first day of college. If you are a homeschooler, you could write about the first day of getting together with other homeschoolers.
I enjoyed my first day of school! I don't remember all the details. However, I do remember that I had been waiting for months to be able to finally go to school. My husband tells me that his first day of school was not so fun. What about you? What was your first day of school like?