[Avodah] The Torah was not given on Shavuos!!

[Shmuel Weidberg]

Surprisingly, the Torah was not given on Shavuos. The Torah was
supposed to be given on the 50th day of the omer, but Moshe added one
day so it was given on the 51st day of the omer which according to the
chachamim was the sixth day of Sivan, because that year both Nissan
and Iyar were Malei. However we celebrate Shavuos on the 50th day of
the omer and it is only because we have a set calendar that Shavuos
always falls on the sixth of Sivan which was the day of Matan Torah.
This is the Chok Yaakov's answer to the Magen Avrahams kushya in OC
Siman Taf tzadik dalet.

So it comes out that the Torah was NOT given on Shavuos but we
celebrate Shavuos on the day the Torah was given. And this was not
always the case. Before Hillel set the calendar Shavuos was not
necessarily celebrated on the day that the Torah was given!!

Gut Yom Tov,
Shmuel