Creation (3760 BCE)
The 1st day of creation, on which G-d created existence, time, matter, darkness and light, was the 25th of Elul. (Rosh Hashanah, on which we mark “the beginning of Your works”, is actually the 6th day of creation, on which the world attained the potential for the realization of its purpose, with the creation of the first man and woman, Adam and Eve. Rosh Hashanah is therefore the day from which the Jewish calendar begins to count the years of history; the 1st day of creation thus occurred on the 25th of Elul of what is termed -1 from creation.

Links: Parshah Bereishit (Genesis 1:1-6:8) with commentary

Jerusalem Walls Rebuilt (335 BCE)
The rebuilding of the walls of Jerusalem — which had been in ruins since the destruction of the First Temple by the Babylonians 88 years earlier — was completed by Nehemia on Elul 25 of the year 3426 from creation (335 BCE) as related in the Book of Nehemia (ch. 6).

Passing of R. Elazar (2nd century CE)
Passing of the Talmudic sage Rabbi Elazar, son of Rabbi Shimon bar Yochai.

Passing of R. Michel of Zlotchov (1786)
Passing of Rabbi Yechiel Michel of Zlotchov (1721?-1786), disciple of the Baal Shem Tov.