On this day in Mexico as well as in many other Spanish Speaking countries, people play practical jokes on each other. It is the Spanish equivelent of our April Fools' Day. On this day you must not believe anything that you are told, nor must you allow anyone to borrow money from you. Traditionally, money borrowed on December 28 does not have to be repaid.
If you allow yourself to fall victim to making a loan on this day, the person who borrowed the money just may say, “Inocente palomita, que te dejaste engañar” or “Innocent little dove, how you've let yourself be fooled”.
Or, the longer version:
Que te dejaste engañar
Sabiendo que en este día
Nada se debe prestar.”
Innocent little dove
How you've let yourself be fooled
Knowing that on this day
You should lend nothing.
By the way, May I borrow uno Peso, por favor?