Is it permissible to place an advertisement on my website for an online store that sells halal products, given that it may offer discounts on goods during Christian holidays or add special decorations for this holiday on its website?

There is no harm in placing an advertisement for a store whose activity is permissible, and it is not forbidden to deal with it because it offers discounts on certain occasions. Promotion and advertising are for permissible goods. However, regarding placing special decorations for a specific occasion and calling for their dissemination, it is not permissible for you to do so because you would be participating in promoting that occasion by spreading these special decorations.