There are only two contemporary accounts of the 1621 Thanksgiving: First is Edward Winslow's account, which he wrote in a letter dated December 12, 1621. The complete letter was first published in 1622, and is chapter 6 of Mourt's Relation: A Journal of the Pilgrims at Plymouth
"Our corn [i.e. wheat] did prove well, and God be praised, we had a good increase of Indian corn, and our barley indifferent good, but our peas not worth the gathering, for we feared they were too late sown. They came up very well, and blossomed, but the sun parched them in the blossom. Our harvest being gotten in, our governor sent four men on fowling, that so we might after a special manner rejoice together after we had gathered the fruit of our labors. They four in one day killed as much fowl as, with a little help beside, served the company almost a week. At which time, amongst other recreations, we exercised our arms, many of the Indians coming amongst us, and among the rest their greatest king Massasoit, with some ninety men, whom for three days we entertained and feasted, and they went out and killed five deer, which they brought to the plantation and bestowed on our governor, and upon the captain and others. And although it be not always so plentiful as it was at this time with us, yet by the goodness of God, we are so far from want that we often wish you partakers of our plenty."
The second description was written about twenty years after the fact by William Bradford in his History Of Plymouth Plantation. Bradford's History was rediscovered in 1854 after having been taken by British looters during the Revolutionary War. Its discovery prompted a greater American interest in the history of the Pilgrims, which eventually led to Lincoln's decision to make Thanksgiving a holiday. It is also in this account that the Thanksgiving turkey tradition is founded.
"They began now to gather in the small harvest they had, and to fit up their houses and dwellings against winter, being all well recovered in health and strength and had all things in good plenty. For as some were thus employed in affairs abroad, others were exercising in fishing, about cod and bass and other fish, of which they took good store, of which every family had their portion. All the summer there was no want; and now began to come in store of fowl, as winter approached, of which this place did abound when they came first (but afterward decreased by degrees). And besides waterfowl there was great store of wild turkeys, of which they took many, besides venison, etc. Besides they had about a peck of meal a week to a person, or now since harvest, Indian corn to that proportion. Which made many afterwards write so largely of their plenty here to their friends in England, which were not feigned but true reports."
The following is a fairly complete list of the foods available to the Pilgrims during the three-day, 1651 Thanksgiving harvest celebration. As can be seen in the above two quotations, the only foods specifically mentioned by the Pilgrims are: "corn" (wheat, by the Pilgrims usage of the word), Indian corn, barley, peas (if any where spared), "fowl" (Bradford says "waterfowl"), five deer, fish (namely bass and cod), and wild turkey.
FISH: cod, bass, herring, shad, bluefish, and lots of eel.
SEAFOOD: clams, lobsters, mussels, and very small quantities of oysters
BIRDS: wild turkey, goose, duck, crane, swan, partridge, and other miscellaneous waterfowl; they were also known to have occasionally eaten eagles (which "tasted like mutton" according to Winslow in 1623.)
OTHER MEAT: venison (deer), possibly some salt pork or chicken.
GRAIN: wheat flour, Indian corn and corn meal; barley (mainly for beer-making).
FRUITS: raspberries, strawberries, grapes, plums, cherries, blueberries, gooseberries (these would have been dried, as none would have been in season).
VEGETABLES: small quantity of peas, squashes (including pumpkins), beans
NUTS: walnuts, chestnuts, acorns, hickory nuts, ground nuts
HERBS and SEASONINGS: onions, leeks, strawberry leaves, currants, sorrel, yarrow, carvel, brooklime, liverwort, watercress, and flax; from England they brought seeds and probably planted radishes, lettuce, carrots, onions, and cabbage. Olive oil in small quantities may have been brought over, though the Pilgrims had to sell most of their oil and butter before sailing, in order to stay on budget.
OTHER: maple syrup, honey; small quantities of butter, Holland cheese; and eggs.
"Have you cut the wheat in the blowing fields,
The barley, the oats, and the rye,
The golden corn and the pearly rice?
For the winter days are nigh."
"We have reaped them all from shore to shore,
And the grain is safe on the threshing floor."
"Have you gathered the berries from the vine,
And the fruit from the orchard trees?
The dew and the scent from the roses and thyme,
In the hive of the honeybees?"
"The peach and the plum and the apple are ours,
And the honeycomb from the scented flowers."
"The wealth of the snowy cotton field
And the gift of the sugar cane,
The savory herb and the nourishing root—
There has nothing been given in vain."
"We have gathered the harvest from shore to shore,
And the measure is full and brimming o'er."
"Then lift up the head with a song!
And lift up the hand with a gift!
To the ancient Giver of all
The spirit in gratitude lift!
For the joy and the promise of spring,
For the hay and the clover sweet,
The barley, the rye, and the oats,
The rice, and the corn, and the wheat,
The cotton, and sugar, and fruit,
The flowers and the fine honeycomb,
The country so fair and so free,
The blessings and glory of home."