Megillah 29B

ועל הכלאים.

And a public announcement is made concerning the need to uproot any instances of diverse kinds that have grown in the fields.

בשלמא על הכלאים ⁠— דזמן זריעה היא, אלא על השקלים מנלן?

The Talmud asks: Granted, an announcement is made concerning the need to uproot diverse kinds, as the beginning of the month of Adar is a time of sowing. Instances of diverse kinds are already noticeable, and therefore it is a fitting time to deal with the matter. But with regard to the announcement concerning the half-shekels, from where do we derive that it should be made at this point in the year?

אמר רבי טבי אמר רבי יאשיה, דאמר קרא: ״זאת עולת חודש בחדשו״, אמרה תורה: חדש והבא קרבן מתרומה חדשה.

R' Tavi said that R' Yoshiyya said: It is as the verse states: “This is the burnt-offering of each New Moon in its renewal throughout the months of the year” (Numbers 28:14). The Torah says: There is a month in which you must begin to renew and bring the daily and additional offering from animals purchased with the new collections of half-shekels collected that year. Each year a collection is made with which to finance the purchase of communal offerings for the following year. Offerings during that year may be purchased only from collections made for that year.

וכיון דבניסן בעי אקרובי מתרומה חדשה, קדמינן וקרינן באחד באדר, כי היכי דליתו שקלים למקדש.

Elsewhere it is derived through a verbal analogy that the yearly cycle begins with the month of Nisan. And since starting from and during the month of Nisan the offerings must be brought from the new collections of half-shekels, it is necessary to make the collection in the preceding month, i.e., in Adar. Therefore, they advance the reading of Shekalim, and they read it on the first of Adar, in order that the people will be reminded to bring the half-shekels to the Temple in good time.