Moed Katan, Chapter Two, Mishnah 1. This mishnah deals with processing olives to produce oil, and with the question of what may be done in this work on Chol HaMoed.
The process of producing oil:
The olives were placed in a storage area called a ma'atan, and there they would soften.
Afterward they would turn them over with a special stick, so that all of them would soften properly and at the same time.
From there they would bring them into the beis habad (the olive press) to be pressed.
If, after the olives had been turned over and softened, they were not brought into the press to be squeezed, the owner was liable to lose them, since they spoil and rot.
"Mi shehafach es zeisav":
This refers to someone who has already completed the process of softening the olives, and all the olives are now soft. The next stage is bringing them into the press, but he did not manage to do so. The mishnah lists the reasons that prevented him from doing this, close to Yom Tov:
Mourning - a close relative died and he became a mourner, and a mourner is forbidden to do work, and therefore he could not complete the process.
Circumstances beyond his control - something else happened that prevented him from doing it.
"Shehit'uhu po'alim" - he hired workers for the job, and they deceived him and did not show up, and he was left with no one to do it.
Now Chol HaMoed arrives, and he wants to do the work on Chol HaMoed, for otherwise his olives will spoil.
The opinion of Rabbi Yehudah:
The mishnah says that he is permitted to "to'en korah rishonah umaniach l'achar hamo'ed" - he may press only the first pressing, and the rest of the process he will do after Chol HaMoed. That is enough to save the olives from spoiling. He is not permitted to finish the entire process, since once he has begun it he will not suffer a great loss if he does not complete it: perhaps some loss will result, but not a significant one. These are the words of Rabbi Yehudah.
The opinion of Rabbi Yose:
Rabbi Yose disagrees and says: "zolef v'gomer v'gaf k'darko". His statement has three parts:
"Zolef" - this can be explained as pouring the olives into the press or into the olive storage area; alternatively, that he removes the olive oil from the pit in the press house.
"V'gomer" - he may complete the process, until all the oil has come out.
"V'gaf k'darko" - and afterward he seals the vessels in which the olive oil is stored, just as he does all year round.
Rabbi Yose's reasoning: since he was permitted to begin the work because of the mourning, the circumstances beyond his control, or the other things that prevented him from doing it before Yom Tov, once we have permitted him to do it on Chol HaMoed, we also permit him to finish the entire process.