Ambush:
I’ve heard that the reason why the Torah did not tell us the exact day of Matan Torah is so that people should not think that the day of its giving is the only day it needs to be studied, supported, loved & kept. None the less, we still celebrate Shavuos as the day of giving of the Torah. A similar idea applies to Mothers & Fathers day, where there is no “set” day written, but its good idea to have a day to celebrate.