Is it true that the Messenger of Allah, may Allah bless him and grant him peace, did not know about the Day of Ashura until late in his life, and that he did not fast on the ninth (Tasua) after learning about it?

The Prophet, peace and blessings be upon him, did not fast Ashura in imitation of the Jews. Rather, he used to fast it before his arrival among them and before knowing their situation. His agreement with them in fasting it was to win them over and draw them in, just as the wisdom behind facing their Qibla was. The Prophet, peace and blessings be upon him, after having fasted Ashura for ten years and commanded its fasting, said: "If I live until the next year, I shall fast the ninth [of Muharram] as well." This was after their obstinacy became apparent, as he then loved to differ from them. This indicates that the preference for agreeing with the People of the Book and differing from them occurred at two different times and in two different situations. However, the final established state was to differ from them after the truth became clear.