end of the year when they want to clean the lots of older models to make room for the new models. also if u have cold winter months car dealerships are also slower then. also at the end of the month they are willing to negotiate more. i know because i work at a dealership.
Iron x will get rid of most contaminants just still use clay for anything it may miss. I use to clay my 3 cars every 6 months but it get tiring and i dont have the time so i just iron x and clay what it misses.