Much of the calories would come from carbohydrates: whole grain breads made from various cereals i.e. barley, oat, rye. The wealthy could afford bread made primarily of wheat.(Please see /u/Qweniden 's comments below for more information in regards to the following) Beer would also provide many calories. It would be low in alcohol and drank at all times of day. The boiling step in brewing killed bacteria (in the contaminated water), and the alcohol provided for a means of preserving the grain ingredient's calories, like a liquid bread. It would not contain hops, may even contain some spices, and would be pretty much flat.

If you were close to the sea you could get fish. Transporting meat was prohibitively expensive, and would be preserved with salt. In any case, meat would not be a very large part of the diet for most people.

Vegetables were not eaten fresh but boiled, as fresh vegetables were viewed as unhealthy. Pottage was the staple and consisted of a pot of ingredients simmering over the fire for long periods of time. Things like grains, vegetables, and if you're lucky, meat, would be thrown in and served out as needed over many days. Kind of like you always have a crock pot/slow cooker going that you toss stuff into as you acquire it and take servings out of when you're hungry. It would have a porridge/thick stew consistency. Remember: no potatoes. Potatoes are a New World crop and a big "no-no" for the party.

So your party will not likely dazzle your guests' taste buds. Buy some whole grain bread loaf, leave it out a few days to get very hard, then serve that. Buy cabbage, porridge, carrots, some salt, maybe rabbit, then simmer it for 12 hours. It's like you're constantly improvising meals with an empty pantry and all you have is a fire and a pot. Kind of getting the idea? Part of the 'problem' is that the common person would not own an oven, which seriously limits what one can do.

But not everyone was destitute, and there would have been good food around. You can look up recipes for noble meals, but they too may seem a little strange. You will see combinations of sugar, cinnamon, and meats that may not seem quite right to your modern taste buds.

Sources from memory:
Tasting Beer by Randy Mosher
A Time Travelers Guide to Medieval England by Ian Mortimer

The Canterbury Tales is 14th century and I have the perfect link for you. :)

http://www.celtnet.org.uk/recipes/the-forme-of-cury.php is a site offering modern English versions of the recipes in 'The Forme of Cury', the cookbook compiled by Richard II's chefs. The cookbook's dates tie in pretty much perfectly with The Canterbury Tales. Though they are a bit fancier than regular peasant fare the recipes might have been aspirational to the pilgrims... and Chaucer as a member of the royal court would certainly have recognised a lot of them!