This sugya discusses two minhagim that are commonly observed at the onset of the Yom Tov of Shavuos.  Many kehillos wait until nightfall to daven Maariv on Shavuos night, and will not accept Yom Tov early. This is to ensure that the days of Sefira are not shortened by being mekabel Yom Tov early, as that may cause the 7 complete weeks of Sefira to be incomplete (i.e. not Temimos).  While there is normally a Mitzvah of Tosefes Shabbos/Yomtov, does this apply to Erev Shavuos according to the above-mentioned minhag?

Others will daven Maariv early but refrain from reciting Kiddush until after nightfall, for the same reason mentioned above. 

We discuss the concept of Kabolas Shabbos, and whether by being mekabel Shabbos/Yom Tov early, we are in essence shortening the day of Erev Shabbos/Yom Tov.. We then go on to understand the concept of Tosefes Shabbos - and the difference between Tosefes Shabbos and Kabolas Shabbos.