The Talmud Tractate Rosh HaShana 10b teaches that Rabbi Eliezer said that the world was created in the "Jewish" month of Tishrei. However, this statement would only refer to the creation of mankind (Adam HaRishon and Chava) but not to the creation of the world itself. The real birthday of the world, so to speak, is five days earlier: On 25 Elul (this Shabbat).
Talmud Rosh HaShana mentions disputes about the true month of creation. One Rabbi claims that it was Nissan, others say it was Tishrei. One explanation of solving this dispute is the opinion of the Ohr HaChaim (any many further commentators) that G - d thought about a creation in the month of Nissan and in Tishrei or Elul, He took action.
The Gemara (rabbinic discussions) in Rosh HaShana 11a asks: "From where do we know that the world was created in Tishrei ?"
Answer: For it is stated:
And G - d said - Let the land be covered with vegetation - herbage yielding seed, trees of fruit.
This verse implies that at the moment of creation the fields were already covered with vegetation, and the trees already laden with mature fruit.
This is implied in the phrase "be covered with vegetation", which suggests a land thickly overspread with mature vegetation, and in the words "trees of fruit", which connote trees laden with mature fruit (Rashi and Maharsha).
And which month a tree is laden with mature fruit ? In Tishrei.
The Gemara in Rosh HaShana 11a brings a proof:
The newly created world required rain and, indeed, rains fell and plants came forth as it is stated:
And a mist arose from the earthand watered the whole surface of the soil.
We see that creation took place at the rainy season, which comes in Tishrei.
There are many further opinions / commentaries on that Talmud teaching. As a matter of fact, the real meaning of the entire creation process is not known to us. We can interpret the Torah content on the subject but our mind just cannot grasp of what G - d did, why and how. How create our whole universe from NOTHING ? Only creating with THOUGHT !
I think it was Aristotle (described in the "Moreh Nevuchim - Guide for the Perplexed" of the Rambam) who claimed that something had existed before creation and that this something was used in order to create our universe.
What was G - d doing before He created the world including us humans ?
I don't understand the famous statement that He created us, the world, the Jews, in order to show His kindness. It rather sounds like a weak excuse for having to say "I don't know ".