Shavuot begins at sundown
Shavuot is the celebration of the giving of the Torah to the Jewish people, also known as the Festival of First Fruits. The Torah was given by the Lord on Mount Sinai on Shavuot more than 3,300 years ago. Every year on the holiday of Shavuot, Jews renew their acceptance of the gift, and the Lord “re-gives” the Torah.
The giving of the Torah was a far-reaching spiritual event — one that touched the essence of the Jewish soul for all times. Sages have compared it to a wedding between the Lord and the Jewish people. Shavuot also means “oaths,” for on this day the Lord swore eternal devotion to the Jewish people, and they in turn pledged everlasting loyalty to the Lord.
In ancient times, two wheat loaves would be offered in the Holy Temple on Shavuot. It was also at this time that people would begin to bring their first and choicest fruits, to thank the Lord for Israel’s bounty.
See https://www.chabad.org/for more information.