Every other yom-tov is listed with its month and day of the month, but not Shavu’ot. It has to be worked out from Lev. 23:4: “From the day after the day of rest you shall count fifty days”; Shavu’ot is the fiftieth day.
The Pharisees and Sadducees had a long drawn-out difference of opinion concerning the phrase, “the day after the day of rest”.
Was it a Sunday, i.e. the day after the first Shabbat of Pesach, as the Sadducees (like the later Karaites) believed, or, as the Pharisees held (supported by the Septuagint), the second day of Pesach, since a yom-tov is also a day of rest?
The sages followed the second view, which means that Shavu’ot works out as the 6th of Sivan (and also, in the Diaspora, the 7th).
For the Sadducees, Shavu’ot would have a different date each year. The Falashas of Ethiopia had yet another view and they counted the 50 days from the day of rest at the end of Pesach, i.e. the 7th day of the festival.
A further difference between Shavu’ot and the other pilgrim festivals of Pesach and Sukkot is that, Biblically, the others last seven days, whilst Shavu’ot is only one day (two days outside of Israel).
The explanation given by Rabbi Joseph B Soloveitchik is that there is a completeness about Pesach and Sukkot, symbolised by the fact that each lasts a full week (more than a week in the Diaspora).
Shavu’ot is the unfinished symphony of Judaism, the incomplete festival. Rav Soloveitchik argues that God shortened Shavu’ot in order to show that human beings have to complete God’s work of creation by applying the teachings of the Torah, given on Shavu’ot, to the task of making the world complete and perfect.