Unlike Sukkot and Passover, Shavuot consists of only one day, not seven. However, there are special customs that pertain to the days following the festival.

The day after a festival is called Isru Hag, based on the verse: “Bind the festival offering [isru hag] with cords, and thence to the horns of the altar” (Psalms 118:27). This is homiletically interpreted to mean: Tie the days of the festival to the following mundane weekdays. Therefore, this day has a slightly festive nature, and some have the custom to add something special to their meal in its honor.

The six days, from the seventh to the twelfth of Sivan, are “days of compensation” for the festival offerings. By Torah law, Passover and Sukkot are celebrated for seven days, during which all Jews were required to make pilgrimages to the Temple and bring the festival offerings. In contrast, the festival of Shavuot lasts only one day, which made it difficult for everyone to ascend to Jerusalem and bring their offerings in that single day. Therefore, six days were added after the festival during which one could bring his individual Shavuot offerings, to make a total of seven days during which the festival offerings of Shavuot may be brought.

Consequently, it is customary in many Jewish communities not to recite Tahanun during all these days of compensation.

Further reading: For more on the destruction of the Temple and the events that precipitated it, see A Concise Guide to the Sages, p. 322.