It says that Avraham Avinu and the Avos kept kol HaTorah kulah. I assume that includes Shabbos. Why then were klal Yisrael given Shabbos by Hashem when they left Mitzrayim? Didnt they have a mesorah from the Avos to keep Shabbos?
Answer:
While Avraham Avinu kept Shabbos and the rest of the Torah, he was not commanded to do so, aside from a a few specific commandments. This was the crucial aspect of Matan Torah, we became commanded to keep the entire Torah. The Gemara records that this was actually on some level against our will, and that it was not until the times of Mordechai and Ester that we accepted the Torah fully and wholeheartedly.
