(Mishnah): When Rosh Chodesh Adar falls on Shabbos, we read Parshas Shekalim.

If it falls during the week, we read it the previous Shabbos, and the next Shabbos has no special Parshah.

(Gemara - R. Levi citing Reish Lakish): Hash-m saw that in the future, Haman ha'Rasha would weigh silver against (offer it to Achashverosh in exchange for letting him kill) Yisrael. [Hash-m] said, it is good that My children's money precede his money. Therefore, we read Parshas Shekalim early (before Purim).

Question: When [Rosh Chodesh Adar] falls on Erev Shabbos, when do we read [Parshas Shekalim]?

Answer #1 (R. Zeirah): We read it the previous [Shabbos].

Answer #2 (R. Ila citing R. Avahu citing R. Yochanan): We read it the next [Shabbos].

R. Zeirah was looking [incredulously] at R. Ila. [R. Ila] said "why do you look at me? I said what I heard [in the name of R. Yochanan], and you said your own opinion, and you look at me?!"

Observation: A Beraisa supports this [opinion], and a Beraisa supports this [opinion].

Support (for R. Zeirah - Beraisa #1): Which is the first Shabbos? It is the one that Rosh Chodesh Adar falls in [the week starting with that Shabbos], even on Erev Shabbos.

Support (for R. Ila - Shmuel - Beraisa #2): Which is the second Shabbos? It is the one that Purim falls in it, even on Erev Shabbos.

Question (Rav Nachman bar Yakov): Exert yourself [to answer this! If Purim is on Erev Shabbos,] the 15th falls on Shabbos. You cannot read the Megilah, for we read Kisvei ha'Kodesh [on Shabbos] only from Minchah and onwards. They should read Parshas Zachor then (and not on the previous Shabbos)!

Answer: Rav taught "veha'Yamim ha'Eleh Nizkarim v'Na'asim" - the Hazkarah (mention, i.e. Parshas Zachor) must be before the doing (Keri'as ha'Megilah).

Note: The Gemara does not say who gave this answer. It seems that it is Shmuel, who brought Beraisa #2, and he was the Rebbi of Rav Nachman bar Yakov. However, we never find that Shmuel cites Rav. Perhaps R. Ila answered this.

Question (R. Aba brei d'Rav Papi): Exert yourself [to answer this! If the 14th is on Shabbos, (we could read Parshas Zachor that day and the Megilah the next day,) and the Hazkarah precedes the doing!

Answer: Did you not already ask this? [Nowadays that the calendar was fixed,] it is impossible [for the 14th to fall on Shabbos]. And if it is possible (when Beis Din is Mekadesh Rosh Chodesh based on witnesses, the Hazkarah would be) after the doing for cities! (Rather, we read Parshas Zachor on the seventh, which is before anyone reads.)

Note: The Gemara does not say who gave this answer. Perhaps it is R. Yosah, who answered an anonymous question like this above (4b). Perhaps R. Aba brei d'Rav Papi asked that question.