The Talmud (b. Bava Kamma 82b) explains that the reason for the Jewish tradition of reading and studying Torah specifically on Mondays and Thursdays (in addition to Shabbat) is based on Exodus 15:22 – “They traveled three days without finding water….” The aftermath of the three days without “water” (mayim) was an eruption of kvetching (complaining).
Water (mayim) has a remez (hint) meaning beyond the p’shat (plain). It often stands as a metaphor for Torah, as in Isaiah 55:1 – “May all who thirst go to the water.”
The implication, then, is that we need to engage Torah often enough that we never go three consecutive days without hearing it. Thus, we read or hear Torah on Shabbat and reflect on what we’ve read for two days, then read or hear more on Monday. We reflect on that content Monday through Wednesday, and then read another portion on Thursday to carry us until Shabbat.
A further significance to the choice of these two specific days is their Creation account pairing. Each day of the six days of creation is paired with another. Days 2 (Monday) and 5 (Thursday) are the forming and filling of the skies and waters.
In the time of Ezra, it came to be that Monday and Thursday were market days for the Jews. Ezra decreed that on those days, there should be a public reading of Torah. These readings happened in the marketplace where there was an audience already assembled (Nehemiah 8:1-8; y. Megillah 4:1).
So… the tradition goes back as far to Ezra (2500 years), with a foundation in Torah -a thousand years before that.
וּשְׁאַבְתֶּם־מַ֖יִם בְּשָׂשֹׂ֑ון מִמַּעַיְנֵ֖י הַיְשׁוּעָֽה
“Then you will joyfully draw water from the springs of salvation” (Isaiah 12:2).