Shavuos is the fiftieth day after Pesach and not a day of the month. So why do we observe two days of Yom Tov if there was no doubt what day the holiday would be on as the case was with the other holidays? Also, why do we celebrate the day that we became obligated to keep all of the Torah and Mitzvos? Didn’t we have the Torah before already, but in a voluntary form?
