This book refers to:
May 27, 1963
After the Earth is destroyed, the pan-dimensional beings (aka, mice) Frankie and Benjy need to figure out the question to life, the universe, and everything. Since their experiment did not finish, but they know the answer is 42, one of the questions they come up with is, "How many roads must a man walk down?" which is a line from Blowin' in the Wind.