Nisan (or Nissan; Hebrew: נִיסָן, romanized: Nīsān from Akkadian: 𒁈, romanized: Nissāni) in the Babylonian and Hebrew calendars is the month of the barley ripening and first month of spring.

The name of the month is an Akkadian language borrowing, although it ultimately originates in Sumerian nisag "first fruits".

Counting from 1 Tishrei, the civil new year, it would be the seventh month (eighth, in leap year), but in contemporary Jewish culture, both months are viewed as the first and seventh simultaneously, and are referred to as one or the other depending on the specific religious aspects being discussed.

The new moon of Aviv, which in Hebrew means "barley ripening" and by extension "spring season"(Exodus 9:31) is one of the few called both by name and by its number, the first.

These are not Nisan molad times, although the offset necessarily remains constant.

If due to start on Monday, Wednesday or Friday it actually begins on the following day.

The table below lists, for a Jewish year commencing on 23 March, the civil date of the first day of each month.

[15] To find how many days the civil calendar is ahead of the Julian in any year from 301 BCE (the calendar is proleptic [assumed] up to 1582 CE) add 300 to the year, multiply the hundreds by 7, divide by 9 and subtract 4.

When the difference between the calendars changes the calculated value applies on and from 1 March (civil date) for conversions to Julian.

The rules are based on the theory that Maimonides explains in his book Rabbinical Astronomy.