The phrase "holiday of a lifetime" is an idiomatic expression that refers to a vacation or trip that someone takes, which they consider to be exceptional and unforgettable. It's often used to describe a once-in-a-lifetime experience, where the person has saved up for years to take this particular holiday, or it could refer to an extraordinary adventure that is unlikely to ever be repeated again in their lifetime. This phrase can also imply that the trip was so amazing and memorable, that it will always be remembered as one of the best experiences of a person's life.