Celebrating Christmas is a form of appreciation not appropriation. A quick search would have answered your question.
Cultural appropriation is the inappropriate or unacknowledged adoption of an element or elements of one culture or identity by members of another culture or identity. This can be controversial when members of a dominant culture appropriate from minority cultures.
According to critics of the practice, cultural appropriation differs from acculturation, assimilation, or equal cultural exchange in that this appropriation is a form of colonialism. When cultural elements are copied from a minority culture by members of a dominant culture, and these elements are used outside of their original cultural context ─ sometimes even against the expressly stated wishes of members of the originating culture – the practice is often received negatively.
Cultural appropriation is considered harmful by various groups and individuals, including Indigenous people working for cultural preservation, those who advocate for collective intellectual property rights of the originating, minority cultures, and those who have lived or are living under colonial rule. Cultural appropriation can include exploitation of another culture's religious and cultural traditions, dance steps, fashion, symbols, language, and music.
Those who see this appropriation as exploitative state that cultural elements are lost or distorted when they are removed from their originating cultural contexts, and that such displays are disrespectful or even a form of desecration. Cultural elements that may have deep meaning to the original culture may be reduced to "exotic" fashion or toys by those from the dominant culture. Kjerstin Johnson has written that, when this is done, the imitator, "who does not experience that oppression is able to 'play', temporarily, an 'exotic' other, without experiencing any of the daily discriminations faced by other cultures". The academic, musician and journalist Greg Tate argues that appropriation and the "fetishising" of cultures, in fact, alienates those whose culture is being appropriated.
The concept of cultural appropriation has also been heavily criticised. Critics note that the concept is often misunderstood or misapplied by the general public, and that charges of "cultural appropriation" are at times misapplied to situations such as trying food from a different culture or learning about different cultures. Others state that the act of cultural appropriation as it is usually defined does not meaningfully constitute social harm, or the term lacks conceptual coherence. Additionally, the term can set arbitrary limits on intellectual freedom, artists' self-expression, reinforce group divisions, or promote a feeling of enmity or grievance rather than of liberation.