Both are fine grammatically. Although we most often say "Christmas time", in fact so often that may even see it written as one word (Christmastime) And this refers to the Christmas season. (the time near Christmas is one big holiday)

No one ever asks if its Christmas. Everybody knows the answer. However, should you be hungover, or just coming out of a coma you might ask: "Is it already Christmas?"

现在是圣诞节！！！ If you want to ask. It should be:"现在是圣诞节吗？" or "已经是圣诞节了吗？"