The Shabbos before Rosh Chodesh Adar that is closest to the month of Nisan, is [called] Shabbos, Parashas Shekalim. If Rosh Chodesh occurs on Shabbos, then that Shabbos is Parashas Shekalim. We take out three sifrei Torah. We call six persons to the first seifer Torah, for the reading of the weekly portion. In the second seifer Torah we call a seventh person for the reading of Rosh Chodesh, beginning with Uve'yom haShabbos. Then half-kaddish is said¹ is placed next to the second one when the half-kaddish is said. (Mishnah Berurah 685:5) (see Chaper 78:1 and 79:1 above). In the third seifer Torah we read the Maftir of Parashas Shekalim, [Exodus 30:11–16], and the Haftarah of Parashas Shekalim is read. If by mistake they began to read the section of Parashas Shekalim first, they should finish reading it,² as well. (Ibid) and for Maftir they should read the section of Rosh Chodesh, and the Haftarah, too, is that of Shabbos Rosh Chodesh.

footnotes: ¹ The third seifer Torah ² This ruling applies to Shabbos, Parashas Hachodesh

The Shabbos before Purim is [called] Shabbos, Parashas Zachor. The Shabbos before Rosh Chodesh Nisan is Shabbos, Parashas Hachodesh. If Rosh Chodesh Nisan occurs on Shabbos, then that Shabbos is Parashas Hachodesh, and it is governed by the same Halachos as apply when Rosh Chodesh Adar occurs on Shabbos. The Shabbos before Parashas Hachodesh is Shabbos, Parashas Parah.

Some authorities maintain that Parashas Zachor and Parashas Parah³ agree that the obligation to read Parashas Parah is not ordained by Scriptural law. (Ibid 685:15) are obligated to be read, according to Scriptural law, and a minor should not be called for the maftir [reading of those parshiyos]. People living in villages who have no minyan⁴. (Ibid 685:16) are required to go to a place where there is a minyan. If that is impossible, they should at least read these [special portions] with the proper cantillation chant.

footnotes: ³ Many of the later Poskim ⁴ This applies even if they have a seifer Torah