If your father has set his heart on a roast chicken dinner for Christmas, it means that he has a strong desire or preference for this particular meal. It shows that your father is looking forward to having roast chicken as the main dish for the Christmas dinner. This could be because he enjoys the taste and aroma of roast chicken, or it could be a tradition in your family to have roast chicken during the holiday season.
Having his heart set on a roast chicken dinner means that your father has made it clear that this is what he wants and that he would be disappointed if it was not included in the Christmas meal. It's important to consider his preference and try to accommodate it if possible, as it can contribute to making the holiday celebration more enjoyable for him.
Remember, this is just one possible interpretation of your father setting his heart on a roast chicken dinner. Other interpretations may also be valid depending on the context and your family's traditions.