September 8th!
Even though school was a total buzzkill for me, I do have a nostalgic sense of 'school never started until the Tuesday after Labor Day' every time I hear of children returning to school in August.

Since I get to decide when our school starts, that was an easy choice for me.
We've always been very casual, year-round, unschoolers.
As we are transitioning to being a waldorf homeschooling family, it seemed right and appropriate to take the summer off to just *be* (and for mama to do lots of reading and planning in preparation for this change.)
As far as keeping track of books read/field trips and the like, we started on May 1st...since we closed last school year out on April 15th. Wow! I hadn't thought of it that way, we're taking a 4 month summer break. Cool. We also moved at the end of April and are still settling in...lots of big changes all at once.