The last day of sacrifice for Shavuot.
When the Holy Temple stood in Jerusalem and all Israel 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").
1922- Agudat Israel established in Kattowitz, Poland.
1991- The Israeli army launched an operation to evacuate Jews stranded in Ethiopia. Within 33 hours, roughly 15,000 people were rescued and brought to Israel.