The best time of year to buy a horse trailer depends on when you plan to start using your horse trailer. If you want to start using your trailer during the eventing season at the beginning of March, you should start designing your custom horse trailer the summer before - building a custom horse trailer usually takes several months. So, itâ€s a good idea to give yourself time to go through the design process, building stages, and delivery phase. That way, your trailer will be delivered right to your door just in time for the riding season.