I am not completely sure, but I seem to recall reading somewhere that the birth of Christ was originally celebrated on the first day of April. The Catholic church had it moved to the twenty-fifth of December in order to incorporate Winter Solstice celebrations. Furthermore, folk who still celebrated x-mas in April instead of on some day the Holy Church decided would be better suited were ridiculed as foolish. This is how the notion the first of April being called April Fool's Day came into history.

As I said at the start, I cannot verify this right out, but it does make sense in a warped sort of way. Any wondering scholars out there feel free to correct me with a truthful illumination of the facts.