Soul Cake Tuesday
First mentioned as Soul Cake Friday in The Dark Side of the Sun, and scheduled as Soul Cake Thursday in Guards! Guards! , this traditional holiday later became firmly fixed on a Tuesday, presumably for convenience. The Soul Cake Days come after the last half-moon in Sektober. (The Discworld Companion)
The day is anthropomorphised (or possibly avimorphised) in the person of a Soul Cake Duck, which is said to lay chocolate eggs. It is said that Susan Sto Helit has made its acquaintance. Inconveniently for the Duck, Soul Cake Tuesday is also the start of the duck-hunting season. (The Discworld Companion)
A soul cake is a small, round cake made for All Souls' Day in Roundworld (the day after All Saint's Day, and therefore two days after the trick-or-treating festival of Halloween), to be given to children and the poor, and presumably the same breadstuff is baked for Soul Cake Tuesday. In Roundworld, each cake eaten frees a soul from Purgatory - and therefore it is certainly one's Christian duty to consume as many as possible. There is an association here with the Welsh custom of "sin-eating", where at the wake, a volunteer will eat the plate of food-fancies left on the coffin - thus transferring the deceased's burden of sin and guilt and ensuring them free passage in the Afterlife.
The 'Soul Cake Duck' may be reminiscent of the Easter Bunny, another animal which symbolises a holiday and is totally unrelated to the foodstuff eaten on that day - although a duck does lay eggs (which the Easter Bunny is associated with), these are not used in soul cakes, at least in Roundworld.
Easter changes dates every year, although this similarity may be accidental.