- In the month of Adar there are 4 Parshiot that are read after the weekly פרשה each Shabbat. The maftir on the first Shabbat after Rosh Chodesh Adar is Parshat Shekalim. The maftir on the second Shabbat after Rosh Chodesh Adar is Parshat Zachor. The maftir on the third Shabbat after Rosh Chodesh Adar is Parshat Parah. The maftir on the four Shabbat after Rosh Chodesh Adar is Parshat HaChodesh. 
- Parshat Shekalim is to remember the mitzvah to donate half-shkalim in the time of the Bet HaMikdash. Parshat Zachor is to remember the mitzvah of detstroying Amalek in proximity to the story of purim. Parshat Parah is in commemoration of the parah adumah done before the inauguration of Mishkan and serves as a prayer to be purified by Hashem. Parshat HaChodesh is to sanctify the new moon of Nissan (which is the beginning of the year in some respects) even though isn’t the primary sanctification of the moon. 
- If one missed reading the פרשה of the week there’s no make up. 
Calenders of Adar
Here are the four possible permutations of how Adar could turn out on the calendar.
|1 Rosh Chodesh, Shekalim|
On the Shabbat before Rosh Chodesh Adar (Adar Bet in a Leap Year) we read the beginning of Parashat Ki Tisa for maftir because in the times of the Beit Hamikdash they would announce the obligation of machatzit hashekel. 
- For haftara we read vayichrot yehoyada, Melachim Bet Chapter 12, because there is discussion about the collection of shkalim. 
- In places where they have the custom to allow a child to go up to the torah for maftir of shkalim or hachodesh, one need not stop them because they have on whom to rely as long as the child knows who he is blessing and he is at least 6 years old. 
- If the congregation forgot to read Parashat Shkalim and instead read the regular maftir, as long as it is still during the morning, even if they didn't remember until after Mussaf they should take out the torah and read Parashat Shkalim and say the berachot before and after.  The haftara should be read without the berachot. 
- Hearing Parshat Zachor is a biblical mitzvah.  It is read the week before Purim. 
- The mitzvah is to express this commemoration verbally and so the reader and those listening should intent to fulfill the obligation. 
- If one can’t make it to hear Parshat Zachor in a minyan one should read it with the Trup from a Sefer. 
- Some say that the word Zecher (with Segol) and some say Zaycher (with Tzeyreh) and so it’s proper to read it twice.  Some say that one should read the entire פסוק twice  while others say it’s enough to read the words Zecher Amalek or Timcheh Et Zecher Amalek a second time. 
- When reading the פרשה of Zachor in Ki Tetse during the year the minhag is to read it Zaycher (with Tzeyreh). 
- An Ashkenazi shouldn’t hear Parshat Zachor from a Sephardic Baal Koreh and a Sephardi shouldn’t hear the Parshat Zachor from an Ashkenazic Baal Koreh.  and some say that this is only preferable but after the fact one has fulfilled one’s obligation. 
- Some say that one should hear the same dialect of Hebrew one speaks and the minhag is to be concerned for this. 
- It’s important that one understands the general point and one fulfills one’s obligation even if one doesn’t understand every word. 
- Some say that if one misses one word one has not fulfilled one’s obligation  while others hold that as long as one heard the main point of destroying amalek one has fulfilled one’s obligation. 
- Some say that the congregation should read along as the Baal Koreh reads Parshat Zachor, while others argue that one should listen quietly and such is the widespread minhag. 
- If afterwards it’s found out that the Sefer Torah used for Parshat Zachor was pasul one doesn’t have to reread it. 
- Someone who has already fulfilled his obligation can still read Zachor again for a different tzibbur. 
- There are many different views as to whether women are obligated to hear parashat zachor in shul or not. 
- Some poskim writes that a woman may fulfill her obligation of listening to Parshat Zachor by reading it from a Chumash. 
- Some poskim permit taking out a sefer torah special for the women to fulfill their obligation but without a beracha. 
- Children who reached the age of Chinuch should be brought to shul in order to hear Parshat Zachor. 
- Some say Parshat Parah is a biblical mitzvah and some it’s a rabbinic mitzvah. 
- S”A 685:1, Mishna Brurah 685:1. Although most poskim hold that the obligation for shkalim and hachodesh are midirabanan there is the opinion of the Elya Rabba 685 in the name of the Rashba Brachot 13a who maintains that all 4 Parshiyot are biblical obligations
- Mishna Brurah 685:1
- Mishna Brurah 685:2
- Shulchan Aruch 586:1-6
- Yalkut Yosef Moadim page 256 from Gemara Megilla 13b and 19a. see also Yerushalmi Megilla 1:5 where a connection is drawn between the half shekel and Purim.
- Tur and Shulchan Aruch 685:5, gemara Megilla 29b, Chazon Ovadia Purim page 1, Yalkut Yosef Moadim page 256.
- Yalkut Yosef Moadim page 271, Birkei Yosef 284:2, Petach Hadvir 282:11. Sh"t Rivash 25 though says not to allow a child to go up for the aliya.
- Sh"t Ginat Vradim Klal 1:35-36,Sh"t Perach Shoshan Klal 1:8, Sh"t Mishpatei Uziel OC 15
- Yalkut Yosef Moadim page 268, Chazon Ovadia Purim page 21
- S”A 685:7, Terumat Hadeshen 108, Rambam in Sefer Hamitzvot Mitzva 189, Smag Mitzva 116 and Smak Mitzva 147. Chazon Ovadia Purim page 11 explains that although it is from the torah not blessing is recited because Hashem isn't pleased with the downfall of the wicked.
- Yalkut Yosef Moadim page 258. Rashi Megilla 29a explains that this is because we want to read about amalek right before we read about Haman.
- Mishna Brurah 685:14
- Rama 685:7
- Mishna Brurah 685:18
- Moadim UZmanim (vol 2, siman 167), Piskei Teshuvot 685:9
- Kesot HaShulchan 88:22 in the notes, Kitzur S”A 88:22, Piskei Teshuvot 685:9, Sh”t Igrot Moshe 8:20:32 (pg 67). Piskei Shemuot (pg 40) quotes Rav Shlomo Zalman as saying that either minhag is acceptable. Piskei Shemuot (39-40) quotes Rav Chaim Kanievsky as saying it’s enough to repeat a few words.
- Piskei Teshuvot 685:9
- Piskei Teshuvot 685:10, Sh"t Binyan Shlomo 54, Piskei Shemuot quoting Rav Shlomo Zalman and Rav Elyashiv
- Rav Nassim Karlitz quoted in Piskei Shemuot (pg 41). Yalkut Yosef (Kriyat HaTorah pg 111 and Moadim pg 260) and Sh”t Yabia Omer 6:11 write that it’s proper for a Sephardi to hear it from a Sephardi.
- Piskei Teshuvot 685:10. Piskei Shemuot (pg 41) quotes Rav Shlomo Zalman saying that it’s not kavod tzibbur to read parshat zachor several times in different dialects.
- Rav Elyashiv quoted in Piskei Shemuot (pg 34)
- Piskei Teshuvot 685:10, Mikrei Kodesh Purim Siman 7
- Rav Shlomo Zalman Auerbach (Halichot Shlomo 18:2)
- Sh”t Minchat Elazar 2:1 says that everyone should read it along with the Baal Koreh, while Sh”t Yechave Daat 3:53 (as well as Yalkut Yosef Moadim page 259 Meor Yisrael on Megilla 18a, and CHazon Ovadia Purim page 4) and Sh”t Meishiv Dvar OC 47 hold that everyone should listen quietly. Piskei Teshuvot 685:7 writes that the minhag is like the second opinion.
- Halichot Shlomo (Moadim vol 2, 18:4)
- Chazon Ovadia Purim page 5.
- The Sefer Hachinuch Mitzvah 603 posits that since the mitzva of zachor is related to the mitzva to destroy amalek, the mitzva is limited to men who go to war and will fight Amalek. Thus, Torat Chessed 1:37, Arugot Habosem 205, and Divrei Chaim 2:14 accept a minhag of women not to go to shul to hear Parshat Zachor. Rav Moshe Feinstein (quoted in Kovietz Halachos, page 13) agrees. On the other hand, the Binyan Tziyon 2:8, however, quotes Rav Nosson Adler as having held that women are obligated to hear Parshat Zachor. The Yeshuot Malko OC 50, Maharil Diskin (Kuntres Acharon) 5:101, Minchat Elazar 2:1-5, and Chazon Nachum 85 agree. Nitei Gavriel (Purim p. 154) writes that the current minhag is women do go to shul to hear Parshat Zachor. See Sh”t Yechave Daat 1:84.
- Nitei Gavriel (Purim p. 154)
- Yalkut Yosef Kriat Hatorah page 136, Chazon Ovadia Purim page 9, Minchat Yitzchak 9:68. See however, Mikraei Kodesh (Purim, 5), Rav Moshe Feinstein quoted in Moadei Yeshurun on Purim page 47, Kovetz Halachot page 15, Kinyan Torah 7:53, Shraga hameir 6:116, and Rav Elyashiv quoted in Halichot Bat Yisrael page 296 who say that there should be a minyan of men for the reading.
- Nitei Gavriel (Purim p. 156)
- S”A 685:7, Mishna Brurah 685:14