This is a calendar of the best holidays to design t-shirts for each month. Make sure to design for holidays 1-2 months ahead. So in January you would design for holidays in February and March. Make sure to check for trademarks. Do NOT design with anything that is copyrighted (Football teams, movies, books, song lyrics, Disney, Star Wars etc.) These are big niches so you will have to niche down. Do not just put Valentine's Day on a t-shirt, think of a specific phrase or graphic that people would want to wear for Valentine's Day. Good luck.