Shekalim, Chapter One, Mishnah One
In the introduction to the tractate I explained what the half-shekel is and what it was used for. Our mishnah teaches that on the first of Adar they would make public announcements telling people to start preparing their half-shekels. The mishnah also teaches other public events that occur in Adar.
1) On the first of Adar they make a public announcement about the shekels and concerning kilayim.
2) On the fifteenth:
a) they read the Megillah [Esther] in walled cities,
b) and they fix the roads and the streets and the ritual water baths,
c) and they perform all public duties,
d) and they mark the graves,
e) and [messengers] go forth also concerning kilayim.
Section one: On the first of Adar, the month before Nisan (the month in which Pesah falls) they begin to make announcements reminding people to bring their shekels, or more specifically half-shekels. They also announce that people should go out to their fields and vineyards to uproot any kilayim that may have sprung up. Kilayim are diverse seeds which have sprung up in the same area.
Section two: The mishnah now begins to teach things that occur, or begin to occur on the fifteenth of the month. The first thing is that on the fifteenth of the month, people in walled cities read the book of Esther, the Megillah. We will learn much more about this when we learn Tractate Megillah. The reason that the mishnah mentions the date upon which it was read in walled cities is that this date coincides with the other things done in the continuation of the mishnah.
The second thing is that they begin to fix the public roads and ritual baths because Pesah is coming in one month. People would need to travel to Jerusalem and purify themselves in order to take part in the pesah sacrifice. Also, Adar is the beginning of the dry season (actually, it can still rain in Adar). It would have been difficult to fix the roads when the rains were still coming down.
In addition they performed all sorts of other public duties that could not be done during the rainy season.
They would mark graves with lime so that priests could see where the graves were and avoid them. During the winter the lime would wash away. Therefore, during Adar, once the rains had stopped they would reapply the plaster.
Above we learned that on the first of Adar they would announce to people that they should go out and check to make sure that there were no kilayim in their fields. On the fifteenth, they would send out messengers to make sure that this had been done. We can see that this was an issue of great importance to them. The rabbis seem to have been especially concerned about kilayim because one cannot tell from looking at picked grain or grapes whether they grew in a field that had kilayim in it. This is true of other food-related problems as well.