The Four Parshiyot

There are four Special Shabbatot,each of which derives its name from the additional Torah portion to be read that day. Two of the Shabbtot happen in the weeks leading to the festival of Purim and two in the Shabbatot then leading up to Chag Pesach

Shabbat Shekalim

Shabbat Shekalim ("Shabbat of the shekels" Plural form שבת שקלים a type Hebrew currency) requests each adult male Yehudi contribute half of a  shekel for the upkeep of the Tabernacle, or mishkan (משכן). The Torah portion Shemot 30:11-16 (the beginning of Parashah Ki Tisa) is read. This Shabbat takes place on the Shabbat before the 1st of the Hebrew Lunar calendar month of Adar, or on the 1st of Adar itself if it falls on Shabbat. In leap years on the Hebrew calendar, when there are two months of Adar, Shabbat Shekalim is on the Shabbat before the 1st of Adar II (or on the 1st of Adar II itself if it is Shabbat). During this  last Shabbat before entering the Adar which is the month of Purim. The Yehudi are to prepare themselves ahead of  festival of Purim.

Shabbat Zachor ("Shabbat of Remembrance שבת זכור) is the Shabbat immediately preceding Purim. Devarim 25:17-19 (at the end of Parasha Ki Teizei), describing the attack by Amalek, is recounted. There was a tradition from the Talmud that Haman, was descended from Amalek. The portion that is read includes a commandment to remember the attack by Amalek, and therefore at this Festival.

Shabbat Parah ("Shabbat of  Red Heifer" שבת פרה) takes place on the Shabbat preceding Shabbat HaChodesh, in preparation for Pesach. Bamidbar 19:1-22 (the beginning of Parashah Chukat) describes the Parah Adumah ("red heifer") in the Jewish temple as part of the manner in which the kohanim and the Am Yehudi purified themselves so that they would be ready ("pure") to sacrifice the korban Pesach.

Shabbat HaChodesh ("Shabbat of the Month" שבת החודש) takes place on the Shabbat preceding the first of the Hebrew month of Nisan (or on the 1st of Nisan itself if it falls on Shabbat), during which Pesach is celebrated. Shemot 12:1-20 (from Parasha Bo) and the laws of Pesach. On the first day of Nisan, HaShem presented the first commandment of how to "sanctify the new moon" (kiddush haChodesh) for the onset of Rosh Chodesh and thus Nisan becomes the first month of the Jewish year.

Shabbat HaGadol

Shabbat HaGadol ("Great Shabbat" שבת הגדול) is the Shabbat immediately before Pesach. There is a special Haftarah reading on this Shabbat of the book of Malachi.

Various reasons are given for the name of this Shabbat:

1. The Midrash Rabbah states: “When they (the Am Yehudi) set aside their paschal lamb on that Shabbat, the first-born goyim gathered near the Yisraelites and asked them why they were doing this. The following was their response: “This is a Pesach offering to HaShem who will kill the firstborn Mitzrayim.” They (the firstborn) went to their fathers and to Pharaoh to request that they grant permission to send the Am Yehudi free – but they refused. The first-born then waged a war against them and many of Mitzrayim were killed. This is the meaning of the verse (Tehillim 136:10): “Who struck Mitzrayim through its first born; for His kindness is eternal”.
2. The Tur states: The lamb was the Mitzrayim deity. Many Yehudim, after 210 years of immersion within Mitzrites civilization, had also adopted this animal as their HaShem. When HaShem commanded that a lamb be set aside and tied to the bed for four days in anticipation of sacrifice, the Am Yehudi abandoned their idolatrous practice and courageously fulfilled this mitzvah in the eyes of the Mitzrites, thereby demonstrating their complete trust and faith in HaShem. Nothing could have been more abominable to the Mitzrites, for their HaShem was to be slaughtered. Nevertheless, miraculously the Mitzrayim were unable to utter a word or lift a hand. They watched helplessly as their HaShem was being prepared for slaughter. This miracle was a nes Gadol (great miracle).
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4. The Peri Hadash writes: On this day the Am Yehudi were commanded to fulfill their first mitzvah – to set aside the lamb as a sacrifice. (Note: The mitzvah of Rosh Chodesh was not one they practically fulfilled at that time on that month.) This significant achievement is therefore called Gadol / Great. Additionally, by fulfilling this first mitzvah they became like a child-like to spiritual Maturity into adulthood – they celebrated their Bar/Bat Mitzvah. In this light, the name Shabbat HaGadol would translate: The Shabbat the Yehudim became gadol/mature adults.
5. The Hatam Sofer writes: On this day the Am Yehudi fully  Teshuvah (returned, repent) to their commitment and faith in HaShem . HaShem is called gadol. Therefore, the Am Yehudi who embraced and subjugated themselves to HaShem earned the title gadol as well.
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7. The Shibolei Haleket writes: The customary lengthy Shabbat HaGadol speech makes the Shabbat feel long, drawn out, and ‘gadol’. (A similar reason is given for Yom Kippur being called Tzoma Rabba / The Big Fast – it feels long!).
8. Rabbi David Ben Yosef  Abudarham writes: In the Haftorah of the Shabbat prior to Pesach we read the possuk [Malachi 3:23]: “Hinei Anochi Shole’ach Lachem Et Eliyahu Hanavi Lifnei Bo Yom HaGadol V’hanorah,”, or "Behold, I send you Eliyahu HaNavi before the coming of the great and awesome day of the Adonai." This reason places Shabbat HaGadol in the same category as Shabbat Hazon, Shabbat Nahamu, and Shabbat Shuva for their name is derived from the Haftarah.
9. Every Shabbat preceding a festival or Chag Moed is known as Shabbat Hagadol. (Shibolei Haleket)