The Torah enjoins us to count the Omer for agricultural reasons [Leviticus 23:15-17]. Later, the counting was reinterpreted as eager anticipation for the giving of the Torah on Shavuot.
(1) What is the earliest written source of this reinterpretation?
(2) Did the Israelites know the Torah was going to be given at a specific date, so they could count in anticipation? If so, what is the source?