First you must be lord of a castle or town (you already have that covered). Second, you must be married. Talk to your wife about holding a feast and she will give you a summary of your ability to hold a feast based on the contents of your larder. You will likely have to travel around a lot to fetch different food and drink to serve in the party.
For the most effective feast you need a large quantity of foods with a wide variety, significant amounts of oil, wine, ale. You will also want to stock up on spice. Once you are ready, tell your wife to send out the invitations.
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