Siman: 685, Seif: 5

So we gave the scenario of Rosh Chodesh (new month) Adar falling out on Shabbat. We gave you a five-Shabbat cycle. If, let’s say, Rosh Chodesh Adar that’s close to Nisan fell during the week, then you have to read the Shekalim on the Shabbat before Rosh Chodesh. That is, even if Rosh Chodesh is on Friday, you read it a week before, on the Shabbat before. Then the second week you have a bye. The second Shabbat, there’s no reading. Why? Because you want Zachor—Zachor to be the Shabbat right before Purim. In the case we just discussed, you need to skip one Shabbat in order to make the Shabbat Zachor close to Purim. And if you have Purim on the first Shabbat—let’s say Purim comes out on a Friday, which can happen—then you have to read the Zachor a week before, always placing it before Purim itself.