Parshas Sheqalim

The Medrish says that the giving of the Machtzis Ha’sheqal was an atonement for the sin of the golden calf.
At the giving of the Torah Hashem threatened Klal Yisroel if they won’t accept the Torah they will be buried under the mountain. The question is asked: Klal Yisroel had said naaseh vnishma, what was the need of Hashem forcing the nation to accept the Torah? One of the answers that is given, is that Klal Yisroel accepted Torah shel bekesav- the written Torah, but were reluctant- and thus had to be forced- to accept Torah shel bal peh- the oral Torah. The Bobover Rebbi explains that the reluctance to accept the oral Torah is what led to the sin of the eigal. When Klal Yisroel saw that Moshe was in heaven for forty days they got scared that he won’t return. Their fear was based that they knew Moshe was the one that brought down the oral Torah but now that he was gone they where scared that that was the end of the process of the Written Torah, and all that was left for them was the Oral Torah which they weren’t so excited to accept and work on. It was this that led them to the grave mistake of the eigal. The atonement was that when they gave a Half Sheqal, they showed Hashem that they were willing to give and toil with, their half of Torah- Torah shel baal peh.
The Oral Torah which was forced upon us at Har Sinai was fully accepted by Klal Yisroel after the miracle of Purim. Klal Yisroel expressing their gratitude to Hashem over the miracle took upon themselves the learning of Torah shel baal peh. Thus leading into the period of the Anshe Kenesses Hagdolah- The Men of the Great Assembly- who were the ones that started the Mishna and Torah shel baal peh (see the 1st mishna in Avos). That is why we read Parshas Sheqalem before Purim for the miracle of Purim is when we expressed our willingness to give Hashem our portion of the Torah- the Oral Torah.