question #3 Sahih Muslim 855a, Book 7, Hadith 28 : What guidance did Allah provide to the Muslim Ummah regarding the observance of a specific day?

We who are the last shall be the first on the Day of Resurrection, except that every Ummah was given the Book before us and we were given it after them. It was this day which Allah prescribed for us and guided us to it and the people came after us with regard to it, the Jews observing the next day and the Christians the day following that.