Related על מגילה 3:4

[When] Rosh Chodesh Adar falls on Shabbat, we read the portion of "Shekalim" (Ex. 30:11-16) and in addition we read the portion of the shekalim that concerns Yehoyada the Kohen (commencing with 2 Kings 12:8). What is the "first Shabbat"? Any Rosh Chodesh Adar falls within [that designation], even on Erev Shabbat. On the second Shabbat, [we read the portion beginning] "Remember [what Amalek did]" (Deut. 25:17) and additionally we read "Thus said the Lord of Hosts, I remember what Amelek did to Israel" (1 Sam. 15:2). What is the "second Shabbat"? Every [Shabbat] in which Purim falls within it, and even on Erev Shabbat. On the third [Sabbath we read the portion of] the Red Heifer, and additionally we read "And I will sprinkle pure water on you and you will be purified" (Ezek. 36:25). What is the "third Shabbat"? Any [Shabbat] that immediately follows Purim. The fourth [Shabbat we read], "This month shall be to you the head of the months" (Ex. 12:2), and in addition [we read] "Thus said the Lord, in the first [month], on the first of the month" (Ezek. 45:18). Which is the fourth Shabbat? Any [Shabbat] in which Rosh Chodesh Nissan falls within it, and even on Erev Shabbat.