Unspayed females run a serious risk of developing pyrometra. Often times this is fatal as the condition is detected too late. The older the female...the more often she's come into heat and the risk is increased.
ACVS - Pyometra in Dogs & Cats
There is a much higher risk of mammary cancer as well.
Most vets wouldn't spay a bitch in heat so you could arrange it way after the holidays.