depends on where you live but in Jersey they will be planted in January. Of course unless you live in Jersey you cant call them Jersey potatoes as they are EU protected and I doubt you will find seed potatoes outside of Jersey for planting. There are plenty of other new potato types that can be grown elsewhere.
The Jersey potato is different also due to the way it is grown and the climate so they may taste very different if grown elsewhere.
For fertiliser just use a balanced granular feed dug into the soil before planting. Potatoes dont like to be over-fertilised.
You'd need to buy seed potatoes called 'International Kidney'. These are actually what Jersey Royals are. The difference in flavour is because, traditionally, seaweed is used as the fertiliser in Jersey. In Jersey they are lifted as 'early potatoes' while they are still immature. If left in the ground, they will grow much bigger, and they end up losing their flavour. In the mainland UK, planting is usually done between February and the end of April, depending on local climatic conditions.