Santa goes to the party at his senpai's house, but she denies ever calling.
But hey, there's a party with friends, come on in.
But Santa puts two and two together: Mai used her magic to help him, and he dropped her like a hot rock once he had a better opportunity.
Santa feel guilty and goes searching for Mai instead. He finds her, moping, at the same place they met at the start.
He apologizes and returns her magic with a kiss, right at the stroke of midnight.