I have a tradition from my Master [the Baal Shem Tov]
that whether Shavuot falls out on the [same] day on which our ancestors received
the Torah, in the time when they sanctified the moon according to visual
testimony,1 or on the seventh [of the month of Sivan] according to
Rabbi Yossi,2 that during the reading of the Ten Commandments, one should
concentrate on the Divine Name derived from the verse: "Open my eyes, that I may
behold wondrous things in Your Torah" (Psalms 119:18) [interspersed with the
Name Y-H-V-H]: (Siddur Rav Shabsai)
One must be extremely careful not to speak a single
idle word from the recitation of the Tikun of Shavuot night 3
night
until after the Kedusha prayer of Musaf.4
For then, all 24
Ornaments of the Bride ascend.
(See Rashi on Exodus 31:18) These ornaments
correspond to the 24 books of Scripture
that comprise the main part of the Shavuot night Tikun, as well as the 24
possible combinations of G·d's holy Name Ado-nai that correspond to the
sefira of malchut. By reciting the Tikun of Shavuos night, one
clothes the Bride - the Divine Presence - in beautiful garments that make her
fit for union with the King.
Thus, one should be very careful to purify their
thoughts at least until this time. (Siddur Rav Shabsai)
[From Sefer Baal Shem Tov; translation and
commentary by Rabbi Dr. Eliezer Shore;
First posted on baalshemtov.com]