[Avodah] Kinyan on Shabbas? (Har Sinai)

[AES]

RHB asked several questions, including the following:
....
2. Kinyan Questions:
A.  Did Hashem make us his nation at Har Sinai (or at Yetzias
Mitzrayim?). If at Har Sinai, isn't that considered a Kinyan on the
part of Hashem??
...
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IIRC, this kinyan did not take place at Har Sinai, but rather occurred
when Hashem told us "You shall be a Kingdom of Priests and a Holy
Nation" and we responded with "Everything that Hashem has commanded we
will do."  Shemos 19:6-8.  According to the Taamei HaMinhagim (brought
down here: http://revach.net/article.php?id=3793&style=print), this
happened on the Yom HaMeyuchas - today - the second day of Sivan.

(However, see http://www.ou.org/chagim/roshchodesh/sivan/yomhameyuchas.htm
where it states that Hashem's statement to us occurred on Rosh Chodesh
Sivan and our response was the next day.)

KT,
Aryeh