In my experience doing water fasts it is better to start a fast after a day of light eating so I wouldn't think a Thanksgiving meal would be good just prior to the fast. For one thing it's to easy to gorge yourself at that meal.
Also doing a Hulda Clarke Liver cleanse prior to the fast would be good for you since you have the constipation problem. The reason being the first item on the cleanse is drinking 4 doses of Epsom salt in water, or juice. The idea is to clean the colon so the liver stones, etc. won't be hindered when you drink the olive oil solution that follows. There is a HC link for the recipe with instructions. It should release the constipation.