So what are the origins of this day? Today I heard a nice little theory, obviously it's not true but I thought it was nice. The day is April 1st, right? Jump 8 months and about 3 weeks forward and you arrive at December 25th, the birthday of Jesus. So on April 1st, when Mary told her friends that she was impregnated by the holy spirit, everyone laughed at her and from that day April 1st is known as April's Fools day.