Should I hold my breath in case of a decompression? If I'm, for example, in the lavatory where there are no oxygen masks, would it be advisable to hold your breath until you can reach an oxygen mask, or until the pilot brings the aircraft down to an altitude where we can breath again?

Is it even physically possible to hold your breath after a  decompression accident?
No. Holding your breath might cause breaking blood vessels in your lung (barotrauma). It is important to equalize the pressure inside your lung with the exterior. That is in fact the most fundamental rule for all scuba divers.

Also see this related question: Is it possible to suffer barotrauma during decompression?