The opening Mishna in Masechta Megilla reads as follows:
מְגִילָּה נִקְרֵאת בְּאַחַד עָשָׂר, בִּשְׁנֵים עָשָׂר, בִּשְׁלֹשָׁה עָשָׂר, בְּאַרְבָּעָה עָשָׂר, בַּחֲמִשָּׁה עָשָׂר. לֹא פָּחוֹת, וְלֹא יוֹתֵר. כְּרַכִּין הַמּוּקָּפִין חוֹמָה מִימוֹת יְהוֹשֻׁעַ בִּן נוּן — קוֹרִין בַּחֲמִשָּׁה עָשָׂר, כְּפָרִים וַעֲיָירוֹת גְּדוֹלוֹת — קוֹרִין בְּאַרְבָּעָה עָשָׂר, אֶלָּא שֶׁהַכְּפָרִים מַקְדִּימִין לְיוֹם הַכְּנִיסָה.
כֵּיצַד. חָל לִהְיוֹת יוֹם אַרְבָּעָה עָשָׂר בַּשֵּׁנִי, כְּפָרִים וַעֲיָרוֹת גְּדוֹלוֹת קוֹרִין בּוֹ בַיּוֹם, וּמֻקָּפוֹת חוֹמָה לְמָחָר. חָל לִהְיוֹת בַּשְּׁלִישִׁי אוֹ בָּרְבִיעִי, כְּפָרִים מַקְדִּימִין לְיוֹם הַכְּנִיסָה וַעֲיָרוֹת גְּדוֹלוֹת קוֹרִין בּוֹ בַיּוֹם, וּמֻקָּפוֹת חוֹמָה לְמָחָר. חָל לִהְיוֹת בָּחֲמִישִׁי, כְּפָרִים וַעֲיָרוֹת גְּדוֹלוֹת קוֹרִין בּוֹ בַיּוֹם, וּמֻקָּפוֹת חוֹמָה לְמָחָר. חָל לִהְיוֹת עֶרֶב שַׁבָּת, כְּפָרִים מַקְדִּימִין לְיוֹם הַכְּנִיסָה, וַעֲיָרוֹת גְּדוֹלוֹת וּמֻקָּפוֹת חוֹמָה קוֹרִין בּוֹ בַיּוֹם. חָל לִהְיוֹת בְּשַׁבָּת, כְּפָרִים וַעֲיָרוֹת גְּדוֹלוֹת מַקְדִּימִין וְקוֹרִין לְיוֹם הַכְּנִיסָה, וּמֻקָּפוֹת חוֹמָה לְמָחָר. חָל לִהְיוֹת אַחַר הַשַּׁבָּת, כְּפָרִים מַקְדִּימִין לְיוֹם הַכְּנִיסָה, וַעֲיָרוֹת גְּדוֹלוֹת קוֹרִין בּוֹ בַיּוֹם, וּמֻקָּפוֹת חוֹמָה לְמָחָר:
Translation: The Megilla is read on the eleventh, on the twelfth, on the thirteenth, on the fourteenth, or on the fifteenth of the month of Adar, not earlier and not later. The mishna explains the circumstances when the Megilla is read on each of these days. Cities [kerakin] that have been surrounded by a wall since the days of Joshua, son of Nun, read the Megilla on the fifteenth of Adar, whereas villages and large towns that have not been walled since the days of Joshua, son of Nun, read it on the fourteenth. However, the Sages instituted that the villages may advance their reading to the day of assembly, i.e., Monday or Thursday, when the rabbinical courts are in session and the Torah is read publicly, and the villagers therefore come to the larger towns. How so? If the fourteenth of Adar occurs on Monday, the villages and large towns read it on that day, and the walled cities read it on the next day, the fifteenth. If the fourteenth occurs on Tuesday or Wednesday, the villages advance their reading to the day of assembly, i.e., Monday, the twelfth or thirteenth of Adar; the large towns read it on that day, i.e., the fourteenth of Adar, and the walled cities read it on the next day, the fifteenth. If the fourteenth occurs on Thursday, the villages and large towns read it on that day, the fourteenth, and the walled cities read it on the next day, the fifteenth. If the fourteenth occurs on Shabbat eve, the villages advance their reading to the day of assembly, i.e., Thursday, the thirteenth of Adar; and the large towns and the walled cities read it on that day, i.e., the fourteenth of Adar. Even the walled cities read the Megilla on the fourteenth rather than on the fifteenth, as they do not read it on Shabbat. If the fourteenth occurs on Shabbat, both the villages and large towns advance their reading to the day of assembly, i.e., Thursday, the twelfth of Adar; and the walled cities read it on the day after Purim, the fifteenth. If the fourteenth occurs on Sunday, the villages advance their reading to the day of assembly, i.e., Thursday, the eleventh of Adar; and the large towns read it on that day, i.e., the fourteenth of Adar; and the walled cities read it on the next day, the fifteenth.
Question: Why by שבת does the משנה say "מקדימין וקורין but by all the other days it just says "מקדימין"
