Taanis, Chapter 1, Mishnah 4. As background to this mishnah, note that the Tanna'im disagree about the timing of the first rainy season - the yoreh mentioned in Shema, the early rains that fall in Marcheshvan. What are the appropriate times for these rains, and when are they supposed to fall?
Three dates are mentioned throughout the mishnah, and each of the Tanna'im holds according to his own opinion. Our mishnah follows the view of Rabbi Meir, according to whom the three times for the first system of rains are:
The third of Marcheshvan.
The seventh of Marcheshvan.
The seventeenth of Marcheshvan.
Our mishnah deals with the question of what the law is, according to Rabbi Meir, in a case where that date has passed and rain has still not fallen.
The text of the mishnah:
"Higia shivah asar b'Marcheshvan v'lo yardu geshamim" - the seventeenth of Marcheshvan has arrived, and in Eretz Yisrael the rain has still not fallen.
"Hischilu hayechidim mis'anin shalosh taaniyos" - hayechidim, literally "the individuals," is a term for Torah scholars. The Torah scholars therefore begin to fast three fasts: Monday, Thursday and Monday.
"Ochlin v'shosin mishtechshach" - they are permitted to eat and drink in the evening. This is not a twenty four hour fast, but only during the day itself.
What is permitted on these fasts:
"Mutarin bimlachah" - they are permitted to do work and to go about their labor.
"U'rechitzah" - they are permitted to wash.
"V'sichah" - and to anoint themselves.
"U'ne'ilas hasandal" - and to wear shoes.
"V'tashmish hamitah" - and to have marital relations.
All of these things are permitted, and they are the things we practice on our ordinary public fast, that is, on the public fasts held during the course of the year.
A note from the Gemara:
The Gemara notes here that the rule that one fasts only during the day applies as long as a person has not gone to sleep. But once he has gone to sleep in the evening, even if he wakes up in the middle of the night, he may no longer eat, since the assumption is that his fast began at the time he went to sleep - unless he made a condition and stated something about this before going to sleep.
In summary: according to Rabbi Meir, once the seventeenth of Marcheshvan has arrived and rain has not fallen, the individuals - the Torah scholars - begin to fast three fasts on Monday, Thursday and Monday. These fasts apply during the day only, and everything else is permitted on them, just as we practice on the public fasts during the course of the year.