Fulfilment of Shavuot

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 7-day festivals of Passover and Sukkot, Shavuot consist only of one day; hence the additional six days of tashlumin or "fulfillment").

Thus we do not recite the tachnun (confession of sins), 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 . ( chabat.org)

Hoping and praying to meet you all soon!

Regards,

Aletta