During the first part of the month of Sivan, Tahanun is not recited, for several reasons.

The first day of Sivan is Rosh Hodesh, and Tahanun is never recited on Rosh Hodesh.

The second day of Sivan is called Yom HaMeyuhas, the “Day of Distinction,” and this is also a reason not to say Tahanun. Two explanations have been offered for the name “Day of Distinction.”

(1) It is on this day that God said to the children of Israel: “You shall be for Me a kingdom of priests and a holy nation” (Exodus 19:6), and the people of Israel thereby received a special “distinction.”

(2) This day has a particular distinction because it occurs between Rosh Hodesh and the three days before Shavuot, which are called the “three days of demarcation,” during which the Israelites surrounded Mount Sinai and prepared themselves for the receiving of the Torah (Exodus 19:15). Accordingly, it was determined that Tahanun should not be recited on this day either.

As mentioned, the third, fourth, and fifth days of Sivan are known as the “three days of demarcation.” God commanded the children of Israel to demarcate Mount Sinai so no one would ascend, and to encamp around it. They were to use these days to prepare themselves physically and spiritually in a variety of ways to receive the Torah. Even today these are considered festive days, on which Tahanun is not recited.

The sixth and seventh days of Sivan are Shavuot itself. The following day (or in Israel, the seventh of Sivan) is called Isru Hag, and it is considered a festive day.

For the next several days (through the twelfth of Sivan), many communities do not recite Tahanun, because these are “days of compensation” for the festival (see explanation at the end of the section, p. 357).

Further reading: For a description of the days leading up to the receiving of the Torah, see A Concise Guide to the Torah, p. 180.