My guess is that it is collection of letters by people about the historic value of the day. It must be something boring like that otherwise he would have kept it for himself.
But seriously, why the downvotes? I responded to a joke about pretend early 20th century spammers with a joke about pretend early 20th century rickrolling. Are we only allowed to be completely serious here? Did I do harm to the level of discourse here? What's going on?