I would seize on "thanksgiving" as a day to further talk about the gospel message. Thanksgiving? Giving thanks to who? The gods?
To the audience: Ah, you believe in God ..... that's good ,,,,, we are part way there already. Lets talk further about God and His interaction with mankind.
Acts 17 Authorized (King James) Version (AKJV)
22 Then Paul stood in the midst of Mars' hill, and said, Ye men of Athens, I perceive that in all things ye are too superstitious. 23 For as I passed by, and beheld your devotions, I found an altar with this inscription, TO THE UNKNOWN GOD. Whom therefore ye ignorantly worship, him declare I unto you. 24 God that made the world and all things therein, seeing that he is Lord of heaven and earth, dwelleth not in temples made with hands; 25 neither is worshipped with men's hands, as though he needed any thing, seeing he giveth to all life, and breath, and all things; 26 and hath made of one blood all nations of men for to dwell on all the face of the earth, and hath determined the times before appointed, and the bounds of their habitation; 27 that they should seek the Lord, if haply they might feel after him, and find him, though he be not far from every one of us: 28 for in him we live, and move, and have our being; as certain also of your own poets have said, For we are also his offspring. ...
Having a day that is all about the harvest is ideal. A good chance to talk about the great harvest that is to come where the wheat is separated from the chaff and gets stored in to the barn and the chaff is burned up.