"וקראתם בעצם היום הזה מקרא קדש יהיה לכם כל מלאכת עבדה לא תעשו"
“And you shall make a proclamation on this very day; a holy convocation shall there be unto you; any laborious work you shall not do.” (Vayikra 23:21)

QUESTION: In what way is Shavuot halachically different from Shabbat and Yom Tov?

ANSWER: According to Torah a Shabbat or Yom Tov runs from evening to evening. Since it is not exactly clear what constitutes nightfall, we begin Shabbat or Yom Tov at the time of sunset and observe it until three stars appear (tzeit hakochavim). In addition, our Sages have instructed that we should add time to the holiday itself, and thus we start the Shabbat or Yom Tov earlier than sunset and observe it until after nightfall (Yoma 81b). Once a person accepts upon himself the holiness of Shabbat or Yom Tov, he is forbidden to do any work and may recite the evening prayers and Kiddush.

In regard to Shavuot, however, there is ahalachic ruling (Shulchan Aruch HaRav, Orach Chaim 494:2) that one should not recite the Ma’ariv — Evening Prayers — or Kiddush on the night of Shavuot before nightfall “because if one will start early and accept upon himself the holiness of Yom Tov in the evening prayer or Kiddush, (declaring that it is “Chag HaShavuot” — “the Festival of Shavuot”) while it is still day, it is as though he is lacking some of the 49 days of counting that precede this Festival, and the Torah said ‘They shall be seven complete weeks’.” On the other hand, since Shavuot is a festival like all other festivals, one is required to add time from the weekday to the holiday and refrain from doing work even before sunset.

This pasuk is alluding to these two halachot as they pertain to Shavuot: “Ukeratem” — the proclamation that you make in the Ma’ariv prayer and Kiddush regarding the holiness and sanctity of the day — should be “be’etzem hayom hazeh” — “in this very day” — after nightfall and not earlier. In addition, similar to all festivals, “mikra kodesh” — “a holy convocation” — “yiheyeh lachem” — “shall there be unto you,” i.e. in your time — adding from the weekday and ushering in the holiday earlier by “kol melechet avodah lo ta’asu” — “not doing any laborious work.”

(פרדס יוסף על ויקרא, ועי' בנטעי גבריאל פי"ב ובתשובות בסוף הספר)