Why school was closed

Overheard from D, the boy who wants me to change him into a reader:

He and another student were examining one of our new DK books on the Titanic. I commented that last week the last survivor of the Titanic had died recently. "When was that?" D asked.
"Last week, I think," I replied.

Then D turned to his friend and whispered excitedly, "Oh, so that's why we had off from school last week!"

I didn't think it would be worth explaining to him about Teacher's Convention week. :)