Menachos, Chapter 11, Mishnah 3. The central subject of the chapter is the lechem hapanim and the shtei halechem, and in the previous Mishnah we learned - at least according to the first opinion, which is the halachah - that the baking of the lechem hapanim and the shtei halechem does not override Shabbos, even though they have a fixed time: the shtei halechem on Shavuos, and the lechem hapanim every single Shabbos.
Our Mishnah contrasts this with the chavitei kohen gadol, which are likewise an offering brought every single day - and nevertheless they do override Shabbos, and their baking on the griddle is done on Shabbos itself. It is because of this contrast that the Mishnah discusses them here.
The chavitei kohen gadol and the half-issaron measure:
The offering is called "chavitin" because it is prepared on a machavas, which is a type of pan. The Kohen Gadol brings all of the fine flour at once - a full issaron - but it is divided into two halves by means of a separate measuring vessel, the half-issaron measure. Altogether twelve loaves are made, half of them offered in the morning and half in the afternoon.
We learned earlier about a dispute as to how many dry measuring vessels there were in the Mikdash - two or three - and whether they were sanctified. Here everyone agrees: even if they disagreed about the measuring vessel of the full issaron, whether it is sanctified as a kli shareis and sanctifies the flour that enters it, regarding the half-issaron measuring vessel used for the chavitei kohen gadol all agree that it is sanctified as a kli shareis, and it therefore sanctifies the flour that enters it with kedushas haguf from the outset.
The reason for the distinction between the vessels:
It is worth noting that the Bartenura in the previous Mishnah wrote that he does not know the reason - in line with the Rambam's commentary - why the beraisa said that the kneading and shaping may be done outside the designated place, while the baking must be done in the designated place. Apparently he did not have before him the version found in our Gemara, since our Gemara does explain the reason: because the vessels sanctify.
The reason the issaron measuring vessel was not sanctified is that every individual would bring menachos, and they did not want a mishap to occur in which the minchah would be taken outside its boundary and become disqualified as yotzei. This reason does not apply to the chavitei kohen gadol: the half-issaron measure was used for the chavitin alone, and those who handle them are kohanim - whether the Kohen Gadol himself or another kohen in his place. They know their work and will not take the minchah outside its boundary, and therefore all agree that this vessel was sanctified and sanctifies immediately.
The wording of the Mishnah:
"Chavitei kohen gadol, lisasan va'arichasan va'afiyasan bifnim" - all three tasks are performed within the Azarah, since the flour enters the half-issaron measure, and because that vessel is sanctified, had the work been done outside, and even if it were taken out for a moment, it would become disqualified as yotzei.
"Vedochos es haShabbos" - the baking on the griddle is done on Shabbos itself, in contrast to the previous Mishnah. The reason: had they been prepared before Shabbos, the disqualification of linah would set in, and they would become disqualified at dawn.
Tosafos note here an additional and separate point: even apart from the flour that becomes sanctified upon entering the half-issaron measuring vessel, the chavitei kohen gadol contain oil. The oil certainly enters the liquid measuring vessel - the measure of three lugin - and becomes sanctified. It follows that from the moment the oil is mixed into the batter, the disqualification of linah certainly applies to it. Therefore, even though we had no clear answer as to why, according to the Tanna Kamma in the previous Mishnah, there is a problem of yotzei while the problem of linah perhaps does not exist, here - even if we set the flour aside on its own - there is certainly a problem of linah on account of the oil, and therefore the chavitin certainly override Shabbos.
Rabbi Akiva's General Rule:
The Mishnah continues: "Kelal amar Rabbi Akiva" - Rabbi Akiva stated a general rule. This rule appears three times in Shas, and it divides into two halves:
"Kol melachah she'efshar la'asosah me'erev Shabbos - einah dochah es haShabbos" - any labor that can be performed on Friday does not override Shabbos. This refers to a labor done for the sake of a mitzvah, such as preparing the chavitin of the Kohen Gadol. A labor that can be done in advance, like grinding the flour, must be done while it is still day, and it does not override Shabbos.
"Vesha'i efshar la'asosah me'erev Shabbos - dochah es haShabbos" - and one that cannot be performed on Friday does override Shabbos. A labor that cannot be done in advance, such as the kneading and baking of the chavitin of the Kohen Gadol, which cannot be done on Friday because of the disqualification of linah (leaving the offering overnight), does override Shabbos.
As mentioned, this rule appears three times in Shas: here, in chapter nineteen of Maseches Shabbos regarding bris milah, and in Maseches Pesachim regarding the korban Pesach.
This is the accepted halachah: even when a mitzvah has a fixed time, as with an offering brought at its set time, that does not permit all the labors surrounding it. Only what must override Shabbos does so, while anything that can be done while it is still day must be done in advance.