התקינו נביאים הראשונים עשרים וארבעה משמרות על כל משמר ומשמר היה מעמד בירושלים של כהנים של לוים ושל ישראלים
The mishna explains: Since it is impossible for the entire nation to be present in Jerusalem when the daily offering is brought, the early prophets, Samuel and David, instituted the division of the priests into twenty-four priestly watches, each of which served for approximately one week, twice per year. For each and every priestly watch there was a corresponding non-priestly watch in Jerusalem of priests, Levites, and Israelites who would stand by the communal offerings for that day to represent the community.
הגיע זמן המשמר לעלות כהנים ולוים עולים לירושלים וישראל שבאותו משמר מתכנסין לעריהן וקוראין במעשה בראשית (ואנשי המעמד היו מתענין ארבעה ימים בשבוע מיום ב' ועד יום חמישי ולא היו מתענין ערב שבת מפני כבוד השבת ולא באחד בשבת כדי שלא יצאו ממנוחה ועונג ליגיעה ותענית וימותו)
When the time arrived for the members of a certain priestly watch to ascend, the priests and Levites of that watch would ascend to Jerusalem to perform the Temple service. And as for the Israelites assigned to that priestly watch, some of them went up to Jerusalem, while the rest of them assembled in their towns and read the act of Creation. And the members of the non-priestly watch, who represented the entire community that week, would fast four days a week, from Monday until Thursday. And they would not fast on Shabbat eve, in deference to Shabbat, as they did not wish to start Shabbat while fasting. And they did not fast on Sunday, so as not to go from rest and delight immediately to exertion and fasting, and run the risk that they might die as a result of the abrupt change.
אֵלּוּ הֵן מַעֲמָדוֹת, לְפִי שֶׁנֶּאֱמַר (במדבר כח), צַו אֶת בְּנֵי יִשְׂרָאֵל וְאָמַרְתָּ אֲלֵהֶם אֶת קָרְבָּנִי לַחְמִי, וְכִי הֵיאַךְ קָרְבָּנוֹ שֶׁל אָדָם קָרֵב, וְהוּא אֵינוֹ עוֹמֵד עַל גַּבָּיו, הִתְקִינוּ נְבִיאִים הָרִאשׁוֹנִים עֶשְׂרִים וְאַרְבַּע מִשְׁמָרוֹת. עַל כָּל מִשְׁמָר וּמִשְׁמָר הָיָה מַעֲמָד בִּירוּשָׁלַיִם שֶׁל כֹּהֲנִים, שֶׁל לְוִיִּם, וְשֶׁל יִשְׂרְאֵלִים. הִגִּיעַ זְמַן הַמִּשְׁמָר לַעֲלוֹת, כֹּהֲנִים וּלְוִיִּם עוֹלִים לִירוּשָׁלַיִם, וְיִשְׂרָאֵל שֶׁבְּאוֹתוֹ מִשְׁמָר מִתְכַּנְּסִין לְעָרֵיהֶן וְקוֹרְאִין בְּמַעֲשֵׂה בְרֵאשִׁית:
These are מעמדות, standing men [and this is the cause of their institution]. It is written (Numbers 28:2), "Command the children of Israel, and say unto them, My offering, viz. my bread," &c. How can an offering be brought for a person without his standing near it [at the time of its being sacrificed]? Therefore did the elder prophets institute twenty-four "mishmaroth" [or divisions of orders]; each mishmarah had always a "mahamad" [or section of standing men], composed of cohanim [priests], Levites, and Israelites, stationed at Jerusalem. When it came to the turn of each mishmarah to go up [from their cities to the Temple], the priests and Levites went up to Jerusalem, and the Israelites who belonged to that mishmarah, assembled in [the synagogues of] their cities to read the history of the creation [viz. the 1st chapter of Genesis].