When did God select Israel?

Question

In Parsha Ki Tavo, Deuteronomy 26:17-18 reads:

(17) You have selected the Lord this day, to be your God, and to walk in His ways, and to observe His statutes, His commandments and His ordinances, and to obey Him. (18) And the Lord has selected you this day to be His treasured people, as He spoke to you, and so that you shall observe all His commandments,

What does Moses mean by stating "this day"? And when did God select Israel, and vice versa, was it not during Matan Torah that this was fulfilled?

Answer 1

When did Israel select G-d?

The Siftei Chakhamim explains that the meaning of "Thou hast avouched the L-rd this day to be thy G-d" is:

You have distinguished Him, from the alien gods, etc.

This, according to the Siftei Chakhamim, is also the explanation of when G-d chose the Jewish people to be His people. It was when they accepted the Torah and followed Him. G-d distinguished the Jewish people from the other nations, since they did not accept the Torah etc..

Furthermore, the Daas Zkenim writes:

G–d credits the Jewish people with having traded in any other deities in favour of the invisible Creator, Who had revealed Himself to them at Mount Sinai.

You can view it as a gift: the Jewish people chose G-d from all the other deities they followed. They realised that Hashem is the True One G-d and pushed aside all false deities. In reward for this, G-d made the Jewish people His treasured people (refer to the Daas Zkenim).

Israel had elevated the L-rd G–d to the topmost branch, and the L-rd responded by elevating the Jewish people to the highest ranking nation.

In Maayanah Shel Torah, the HaKesav VeHaKabbalah is quoted in saying that G-d has made the Jewish people exceptional and important by giving them the Torah and mitzvos to keep. This is the distinction that sets them apart from the other "seventy nations".

Answer 2

Refer to the Ohr HaChaim:

את ה׳ האמרת היום להיות. "You have distinguished Hashem this day to be a G-d for you, etc." - The reason the Torah added the word היום, "this day," may be understood in conjunction with what we learned in Kesubos 110 that "anyone who lives in the diaspora is considered as if he did not have a G-d." The words את ה׳ האמרת היום apply only to people who are in the Holy Land. Even though at that precise moment the Israelites were still on the land that used to belong to Sichon and Og, this was considered part of the Holy Land seeing that it had been conquered by the whole nation at the command of their prophet Moses. We have discussed and proved this point in connection with our commentary on Bamidbar 32:3. Seeing that the land in question was no longer part of the diaspora, the Israelites could truly claim to have adopted the Lord as their G'd, and G'd in turn adopted them as His exclusive people. (Sefaria translation and notation)