Knowing the harvest seasons and its grains
They are as follows;
1) Soyabean - September - December
2) Ginger - November - January
3) Rice - October - December
4) Yam - September - December
5) Sesame Seed - January - March
6) Egusi - July - September
7) Corn - September - November
8) Beans - November - December
9) Sorghum - December -January
10) Groundnut - September - October
11) Cashew Nut - January/ February - May
12) Time to get Palm Oil - February - April
Also remember that different regions do harvest their produce at different times and this is because of their ecological zone, they might plant at a different time and surely their harvest won't be at the same time.
It is now left to you to do your research about the state or region which you want to buy from so you can work with their calendar.
Also, when there is a fresh harvest, try as much as possible to buy dry grains, not just dry ones, I mean well dried ones if it's for the purpose of storage, if you don't buy dried grains you will lose all of the money invested into it.