Hallogivmas is the brain child of Mike Essex and Jana Burns, first conceived early in 2010. It grew from a discussion of our favourite things about various holidays: gifts, turkey, and costumes. The end of April was decided to be the best time of year to celebrate since it was placed in a holiday void, but still close enough to Easter to buy turkeys on sale. Also, it would never do to have Hallogivmas celebrated on a weekday, so the last Saturday of April was ultimately chosen.
It's pretty much a holiday for your friends to get together and have a good time.