R. Shaul DovBer Zislin was a student in Yeshivat Tomchei Temimim in Lubavitch. He went on as to serve as the director and spiritual mentor of the yeshiva’s division in Shchedrin, and later as rabbi in Berezin (Byerazino) and Orsha (all three are towns in present-day Belarus). In 1934 he emigrated to the Land of Israel, where he served as rabbi of the Meah Shearim neighborhood of Tel Aviv until his passing on 12 Sivan, 5724 (1964).
R. Shaul DovBer was known as a clever individual with a noble character. He possessed a deep knowledge of Chassidut, and had the unique ability to clarify the most obscure concepts, which attracted many listeners to his lectures and farbrengens.
Link: The Meeting
When the Holy Temple stood in Jerusalem, and all Jews would come there for the three annual "pilgrimage festivals" (Passover, Shavuot and Sukkot), Sivan 12 was the last of the seven days allotted for the offerings brought in conjunction with the Shavuot pilgrimage (unlike the festivals of Passover and Sukkot, which have seven biblically mandated days, Shavuot consists only of one day; hence the additional six days of tashlumin or "fulfillment").
Thus we do not recite the tachnun (confession and penitential suplication), and the other prayers omitted on a festival or joyous commemoration, from the 1st of Sivan until and including the 12th, as all these days bear a connection with the festival of Shavuot (see last week's calendar entries for Sivan 1 through 6).
Links: about the temple Offerings