Is eating Turkish chicken on Thanksgiving, or giving one's wife a gift on Valentine's Day, considered a normal day that violates the Sharia ruling and constitutes committing a sin?

There is no harm for a Muslim in eating chicken or giving his wife a gift on a day that coincides with a holiday of the polytheists, provided that this is not a celebration or observance of their holiday. Whoever does that is like one who eats or honors his family without knowing about the polytheists' holiday.