Likutei Sichos vol 28, Chag Hashavuos, Rabbi Chaim Wolosow
https://www.torahrecordings.com/likutei-sichos/028/004_001 לקוטי שיחות חלק כח - חג השבועות The Rebbe analyzes why the day after Shavuos became “Yom Tavo’ach,” when the Olos Re’iyah were brought. Even according to Beis Hillel, many delayed these korbanos to emphasize the unique simcha and “lachem” of Matan Torah on Shavuos itself.