The holiday isn’t Santa’s to postpone. He is an avatar and caretaker of the Christmas spirit, not a dictator with control over it. Often in the ebin deep lore of these stories, Christmas itself has both power on the specific date which is needed by Santa, and it has a need for the rituals to be completed least it be damaged. Like an Aztec sacrifice to ensure the sun rises, it isn’t just the sort of thing you can delay.