December is a busy month for most faiths, so regardless of your's if you have a developmentally delayed child you'll be wanting to avoid the meltdowns that can happen with so much activity. Flashing lights, loud music, and crowds of people can be a trigger if your child has sensory issues. Put yourself in your child's place, even get down on their level to see exactly what they see. Younger children are down in a sea of people with stimuli bombarding them from every angle. Nonsymptomatic children get fussy in these situations, just imagine what the child with disabilities and sensory sensitivity is going through.
Some tips to avoid overstimulation starts before you leave your home. Talk to your child and explain where you are going, what you are going to do, how many people are going to be present, what's expected of them, and give your child a word or phrase to use to communicate when he's almost on overload. The last item works best with a little older children who can recognize a meltdown coming on. Feed your child before going out. An empty tummy leads to a bad attitude.
When you are out give yourself permission to stop what you are doing, i.e. shopping, dining out, etc., and remove yourself from the situation. Getting angry or upset over a meltdown only adds fuel to the fire. You have to remain calm, the child might begin to mirror your calm behavior. If you do not remain calm you can be guaranteed you will be mirrored resulting in a meltdown of gigantic porportions.
Give your child a responsibility while you are out. If you are shopping make it his job to remember one or two items and watch for them on the shelf. It is difficult to have a meltdown when you are concentrating on something. Make a game of whatever you are doing. If you are playing a game your child is less apt to go off the deep end.
Use a weighted sock, blanket or vest prior to and shortly after a large event. You can find an example at http://weightedvest.com/. You can make the sock yourself. Simply fill a tube sock with white rice and sew the end shut. Either microwave or refrigerate for the desired effect and place the tube sock around your neck laying on your shoulders. I added a vanilla scent to the rice so we had aromatherapy going on at the same time. Lavender and vanilla are the two best relaxers.
These are just a few suggestions. I'll add more later as I recall exactly what we did to reduce our sons meltdowns.