On a typical week, the Torah is read publicly on Monday, Thursday, and twice on Shabbat. On holidays, fast days, and Rosh Chodesh, the Torah is read regardless of the day of the week.
When did this start?
Moses instituted that the Torah be read three days a week. The Talmudic sages1 find this alluded to in Exodus (15:22-27), where we read that our ancestors traveled for three days and thirsted for water—which allegorically also refers to the Torah. They had become spiritually ill after not studying Torah. In response, Moses and the prophets of his generation decided that three days should never pass without a public Torah reading. Thus, we read the Torah on Shabbat, then skip a day and read it on Monday, then skip two days and read it again on Thursday—then two days later we are back at Shabbat.2
At first glance, this would seem to contradict the tradition3 that Ezra the Scribe and the Men of the Great Assembly introduced the practice of reading the Torah on Mondays and Thursdays in the 4th century BCE.
The Talmud reconciles these two traditions by explaining that they refer to different stages in the evolution of this tradition. In Moses' times only three verses were read (corresponding to the three general groups within the Jewish community: Kohen, Levi, and Yisrael) on the weekdays. Ezra and associates lengthened this quota to a minimum of 10 verses (divided into three Aliyot).
Please let me know if this helps.
Yours truly,
Rabbi Baruch S. Davidson
So Monday & Thursday are just intended to make sure the Torah reading is spaced out and there are no long gaps? Is the reading of the Torah on these two extra days also why there is fasting on Mondays & Thursdays? I had heard the Jewish tradition also indicated part of the reason for fasting those days was Moses ascended Mt. Sinai on a Monday and descended on a Thursday - but when I look into that myself, I can't find anything to that effect.
Thank you!
The reason why there is fasting on Monday and Thursday is that since people would drink alcohol on the holidays, they were worried they might have committed a sin while intoxicated, so they fasted on the days following the holidays (Pesach and Sukkos).
On top of that, we add in an extra set of prayers on Mondays and Thursdays, because Moshe ascended and descended on that day of the week. See here.
Some connect the ideas, that the Torah is read, and extra prayers are added, and fasts are done on those days, because those are the days when the heavenly tribunal sits, so it is a good time for repentance.