When is the best time to plant watermelon?
I need to know if there is any existing market for the supply of watermelon.
To answer your first question, watermelon is a warm season crop, so grows in the summer. This article has more information: https://www.gardeningknowhow.com/edible/fruits/watermelon/growing-watermelon.htm
Regarding any question about local markets and supplies, as well as growing information specific to your climate and soil, you can seek information from your regional agricultural services. You should be able to access them through the nearest university or botanical garden.