The following is a clear and coherent cut-and-paste from my local masjid's website.
As April draws near, non believers around the globe celebrate a holiday on the first of the month known as April Fool’s Day. The traditional practice of this day is to play some sort of practical joke on someone and make him the “April Fool”.
Pranks performed on this day range form the simple, (such as saying, “your shoes are untied!”) to the elaborate. Whatever the prank, the trickster usually ends it by yelling to his victim, “April fool!”
The history of April Fool’s Day is not totally clear. No exact date can be found pinpointing the first official celebration of the holiday. There are, however, many narrations regarding the reason the tradition is practiced. One narration states that it is a day the Spanish celebrate in remembrance of the day that they defeated the Muslims and swept them from power in lower Spain. According to this narration, the Christian army could not conquer the Muslims there and sent spies to discover why. They discovered that the Muslims were strong on faith and obedient to Allah. The tradition goes on saying that the Christians sent in alcohol and tobacco, which the Muslims used and became the means in which they lost their obedience to Allah. Then the Christian army invaded lower Spain and conquered the Muslims on the first of April and began celebrating April Fool’s Day ever since. As this story has been circulating between our Muslim brothers strengthening their beliefs about the history of this day I deemed it appropriate to clarify a few points about the history of April Fool’s Day. The truth of the matter is that this story is not true in many aspects.
First, let us confirm the historic fact that Muslim rule in Spain ended on the 2th of January in the Christian year of 1492 A.D. April Fool’s Day was not heard of until over sixty years later. The most authentic reports according to Christian historical books such as Encyclopedia Britannica as well as many others trace the roots of the holiday back to 16th century France. Prior to the year 1582, the new year was celebrated for eight days, starting with the 25th of March. The celebration culminated on April 1st. With the reform of the Christian calendar under King Charles IX, through the influence of Pope Gregory, the Gregorian calendar was introduced, and New Years Day was moved to the first of January. However, due to lack of communications in those days, many people did not receive the news for several years. Others, the most obstinate crowd, refused to accept the new calendar and continued to celebrate the New Year on April 1. These backward people were classified as “fool” by the general public and were often subject to some ridicule. Hence, the tradition of April Fool’s Day began.
Second, we should acknowledge the fact that tobacco products were not even invented until after the time of the fall of Muslims in Spain. According to the State College in Framingham, they quote that smoking of any kind did not exist in Spain until after it was brought back from the New World in the times after King Ferdinand of Spain had come to power. Tobacco use did not become widespread until the 19th century.(Introduction to human biology, Framingham State College, Roger N. Morrissette, Ph.D.)
So regarding the rumor claiming that April Fool’s Day is a celebration of the take over of Muslim Spain, it is clear the whole thing is a fabricated story. It is in fact a lie made by the nonbelievers to mislead Muslims and in realty create a “Muslims Fool Day”! True Muslims would never propagate lies especially when it comes to the history of Islam. It is our duty to educate our brothers on these kinds of topics.