In her History of Earth Day in a Nutshell, Jaymi tells us that
Senator Nelson thought the week of April 19-25 would be ideal for ensuring college students would take part, hoping to achieve an environmental teach-in during a time between exams, spring breaks, and religious holidays. Narrowing it down, he chose Wednesday, April 22. Very practical.
But lots of holidays, like Presidents Day, Labor Day and Thanksgiving, float around for the convenience of citizens. Today people are at work or at school and many don't get to do much about Earth Day.