כֹּה־אָמַר֮ אֲדֹנָ֣י יְהוִה֒ בָּֽרִאשׁוֹן֙ בְּאֶחָ֣ד לַחֹ֔דֶשׁ תִּקַּ֥ח פַּר־בֶּן־בָּקָ֖ר תָּמִ֑ים וְחִטֵּאתָ֖ אֶת־הַמִּקְדָּֽשׁ׃
Thus saith the Lord GOD: In the first month, in the first day of the month, thou shalt take a young bullock without blemish; and thou shalt purify the sanctuary.
Mishnah Megillah
If Rosh Hodesh Adar falls on Shabbat the portion of shekalim is read [on that day]. If it falls in the middle of the week, it is read on the Shabbat before, and on the next Shabbat there is a break. On the second [of the special Shabbatot] they read “Zakhor;” On the third the portion of the red heifer; On the fourth “This month shall be for you;” On the fifth the regular order is resumed. They interrupt [the regular order] for anything: for Rosh Hodesh, for Hanukkah, for Purim, for fasts, for Ma’amadot, and for Yom HaKippurim.