Parshat Shekalim is the Torah Portion we read on the Shabbat before Adar or on the first day of Adar if it comes out on Shabbat.
1. Parshat Shakalim is the first of what is called the four parshiot. The Mishna in Rosh Hashana says: “If the new month of Adar comes out on Shabbat we read Shekalim then. Ifthe new month comes out during the week we read it on the Shabbat beforehand the next Shabbat take a break (from the four parshiot).The second Shabbat we read “Zachor”, the third we read “Parah” and the fourth we read “This month is for you the first of all months” after that we resume reding the regular Shabbat Torah portion.”
2. The main reason we read Shekalim is because in the time of the temple every Jew was obligated to share in the sacrifices of the year. This was done by giving a half Shekel to the temple for this purpose in Adar to buy public sacrifices, the daily sacrifices and the additional ones, the pouring of wine and oil for the gift offerings, and other items. Since Nissan is the New Year for the donation cycle for these sacrifices in the temple they used to collect the half shekel a month before in the beginning of Adar in order to give time to all the Jews to get their half Shekel to the temple. In the Mishna of Shekalim (chapter 1) it says: “On the first of Adar we announce about (the collection of) Shekalim.” This means that we announce and call out informing everyone that the day when the sacrifices are brought from new monies is approaching so everyone should bring their half Shekel to the temple to be counted in and not miss the mitzvah. This ‘announcement’ was the reading of the Torah portion of Shekalim, when everyone was in the synagogue they would hear the Torah portion and expeditiously give their half shekel right away.
3. The Sefer Hachinuch writes(mitzvah 105): “And now with our many sins we no longer have a temple and no Shekalim, the nation of Israel has the custom to remember this mitzvah and read this Torah portion every year.” Additional reasons why we read the Torah portion specifically on Shabbat is mainly because people are home and everyone can gather in the synagogue to hear the reading of the Torah portion whereas on a weekday less people are around.
4. The Admor of Slonim the Netivot Shalom wrote that the four parshiot belong with the Shabbat because through the reading of these Torah portions it is as if we are keeping the part of the mitzvah that corresponds to the soul. The level where speech can be considered action is attained on the day of Shabbat as the verse says; “to make the Shabbat” and this is even though we do not learn out any doing from that verse other than saying Kiddush. Similarly when we read this portion of Shekalim our speech gets elevated on Shabbat to the point that it’s considered that we did the mitzvah.