Menachot 65: The Date of Shavuot

A new Mishna teaches us how to prepare barley for the omer offering brought on the second day of Passover.  Apparently the beit din would send out emissaries just before Pesach and tie the barley that was unreaped into sheaves so that it would be easier to reap.   Every in nearby towns would join them to harvest the barley in celebration.  When it became dark, the reaper would call out a number of questions twice, to which the people would answer "Yes!"

• Has the sun set?
• With this sickle?
• Into this basket?

On Shabbat, there were further questions, each one twice:

• On this Shabbat?
• Shall I cut?
• to which they answered, "Cut!"

The section were then repeated two more times. 

The Gemara explains that this was all done because of a religious sect called the Baitusim who disagreed with the Sages regarding the omer sacrifice.  They interpreted Sefer Vayikra (23:11) as saying that the omer must be brought on the following Shabbat when they refer to the first day of Pesach.  They believed that the omer sacrifice and thus Shauvot was always brought on Sunday.

later, when the Sages decided on halachic clarification, Shavuot was placed in its "proper" time and a minor holiday was added and written in Megillah Ta'anit.  Most holidays and fasts recorded in Ta'anit came from the events that happened during the second temple era.  Some other events (earlier and later) are included).  It is not part of the standard Talmud collection, though Steinsaltz includes as an addendum to Masechet Ta'anit.