Nisan in the context of Babylonian calendar

The Passover Seder is a ritual feast at the beginning of the Jewish holiday of Passover. It is conducted throughout the world at the start of the 15th day of Nisan in the Hebrew calendar (at sunset, when a Hebrew day begins). The day falls in late March or in April of the Gregorian calendar. Passover lasts for seven days in Israel, and customarily usually eight days in the Jewish diaspora. A seder is held on the first night, the 15th of Nisan; where eight days are observed, a seder is often also held on the second night.

The Seder is a ritual involving a retelling of the story of the liberation of the Israelites from slavery in ancient Egypt, taken from the Book of Exodus (Shemot) in the Torah. The Seder itself is based on the Biblical verse commanding Jews to retell the story of the Exodus from Egypt: "You shall tell your child on that day, saying, 'It is because of what the LORD did for me when I came out of Egypt.'" (Exodus 13:8) At the seder, Jews read the text of the Haggadah, an ancient Tannaitic work narrating the Israelite exodus from Egypt, with special blessings and rituals, Talmudic commentaries, and Passover songs.

Joseph ben Ephraim Karo, also spelled Yosef Caro, or Qaro (Hebrew: יוסף קארו; 1488 – March 24, 1575, 13 Nisan 5335 A.M.), was a prominent Sephardic Jewish rabbi renowned as the author of the last great codification of Jewish law, the Beit Yosef, and its popular analogue, the Shulhan Arukh. Karo is regarded as the preeminent halakhic authority of his time, and is often referred to by the honorific titles HaMechaber (Hebrew: הַמְחַבֵּר, lit. 'the author') and Maran (Jewish Babylonian Aramaic: מָרַן, lit. 'our master').

Rosh Hashanah (Hebrew: רֹאשׁ הַשָּׁנָה‎, pronounced [ˌʁoʃ haʃaˈna]; lit. 'head of the year') is the New Year in Judaism. The biblical name for this holiday is Yom Teruah (יוֹם תְּרוּעָה‎, Yōm Tərūʿā, IPA: [joːm təruːˈʕaː]; lit. 'day of blasting'). It is the first of the High Holy Days (יָמִים נוֹרָאִים‎, Yāmīm Nōrāʾīm; lit. 'Days of Awe'), as specified by Leviticus 23:23–25, that occur in the late summer/early autumn of the Northern Hemisphere. Rosh Hashanah begins the Ten Days of Repentance, culminating in Yom Kippur, the day of atonement. It is followed by the festival of Sukkot, which ends with Shemini Atzeret in Israel and Simchat Torah everywhere else.

Rosh Hashanah is a two-day observance and celebration that begins on the first day of Tishrei, which is the seventh month of the ecclesiastical year. The holiday itself follows a lunar calendar and begins the evening prior to the first day. In contrast to the ecclesiastical lunar new year on the first day of the first month Nisan, the spring Passover month which marks Israel's exodus from Egypt, Rosh Hashanah marks the beginning of the civil year, according to the teachings of Judaism, and is the traditional anniversary of the creation of Adam and Eve, the first man and woman according to the Hebrew Bible, as well as the initiation of humanity's role in God's world. The Sages in the Talmud have characterized the day of Rosh Hashanah as the day that we so to speak crown God as king anew each year. This is effectuated through the Shofar blasts which symbolize the horns sounded at a coronation. It also initiates the ten days of repentance making it an opportune time for repentance.

Tishrei (/ˈtɪʃreɪ/; Hebrew: תִּשְׁרֵי‎ [tiʃˈʁej] ) or Tishri (/ˈtɪʃriː/; תִּשְׁרִי‎ [tiʃˈɾi]; from Akkadian tašrītu 'beginning') is the first month of the civil year (which starts on 1 Tishrei) and the seventh month of the ecclesiastical year (which starts on 1 Nisan) in the Hebrew calendar. The name of the month is Babylonian. It is a month of 30 days. Tishrei usually occurs in September–October on the Gregorian calendar.

In the Hebrew Bible the month is called Ethanim (Hebrew: אֵתָנִים – 1 Kings 8:2), or simply the seventh month. In the Babylonian calendar the month is known as Araḫ Tišritum, "Month of Beginning" (of the second half-year).

Akitu or Akitum (Sumerian: 𒀉𒆠𒋾, romanized: a-ki-ti)(Akkadian: 𒀉𒆠𒌈, romanized: akītu(m))is a spring festival and New Year's celebration, held on the first day of the Assyrian and Babylonian Nisan in ancient Mesopotamia and in Assyrian communities around the world, to celebrate the sowing of barley. Akitu originates from the Sumerian spring New Year festival of Zagmuk.

Fast of the Firstborn (Hebrew: תַעֲנִית בְּכוֹרוֹת, romanized: taʿăniṯ bəḵoroṯ or תַעֲנִית בְּכוֹרים taʿăniṯ bəḵorim) is a unique fast day in Judaism, which usually falls on the day before Passover (i.e., 14 Nisan, a month in the Hebrew calendar; Passover begins on 15 Nisan). In modern times, the fast is usually broken at a siyum celebration (typically made after Shaharit), which, according to the prevailing custom, creates an atmosphere of rejoicing that overrides the requirement to continue the fast. Unlike all other Jewish fast days, only firstborn children must fast on this day.

This fast commemorates the salvation of the Israelite firstborns during the Plague of the Firstborn (according to the Book of Exodus, the tenth of the ten plagues wrought upon Biblical Egypt before the Exodus), when, according to Exodus (12:29): "In the middle of the night יהוה struck down all the [male] first-born in the land of Egypt, from the first-born of Pharaoh who sat on the throne to the first-born of the captive who was in the dungeon, and all the first-born of the cattle. "

Iyar (Hebrew: אִייָר‎ or אִיָּר‎, Standard ʾĪyyar Tiberian ʾĪyyār; from Akkadian: 𒌗𒄞 ayari "rosette; blossom") is the eighth month of the civil year (which starts on 1 Tishrei) and the second month of the Jewish religious year (which starts on 1 Nisan) on the Hebrew calendar. The name is Babylonian in origin. It is a month of 29 days. Iyar usually falls in April–May on the Gregorian calendar.

In the Hebrew Bible, before the Babylonian captivity, the month was called Ziv (זו‎, 1 Kings 6:1, 6:37). Ziv is a Hebrew word that means "light" or "glow".

Shevat (Hebrew: שְׁבָט, Standard Šəvaṭ, Tiberian Šeḇāṭ; from Akkadian: Šabātu) is the fifth month of the civil year starting in Tishre (or Tishri) and the eleventh month of the ecclesiastical year on the Hebrew calendar starting in Nisan. It is a month of 30 days. Shevat usually occurs in January–February on the Gregorian calendar.

The name of the month was taken from the Akkadian language during the Babylonian Captivity. The assumed Akkadian origin of the month is Šabātu, meaning "strike", that refers to the heavy rains of the season. In Biblical sources, the month is first mentioned by this name in the book of prophet Zechariah (Zechariah 1:7).

Pesach Sheni (Hebrew: פסח שני, trans. Second Passover) occurs every year on 14 Iyar. This is exactly one month after 14 Nisan, the day before Passover, which was the day prescribed for bringing the Korban Pesach ("Paschal offering", i.e. Passover lamb) in anticipation of that holiday. As described in the source text for this mitzvah (Numbers 9:1–14), the Israelites were about to celebrate Passover one year after leaving Egypt.

The offering of the Korban Pesach was at the core of that celebration. However "certain men" were ritually impure from contact with human corpses, and were therefore ineligible to participate in the Korban Pesach. Faced with the conflict of the requirement to participate in the Korban Pesach and their ineligibility due to impurity, they approached Moses and Aaron for instructions, which resulted in the communication of the law of Pesach Sheni.