To determine the correct date, several pieces of information need to be obtained, using astronomical tables:

(1.) Date and time of the Vernal (Spring) Equinox of the given year.

(2.) Date and time of the New Moon preceding the Equinox.

(3.) Date and time of the New Moon following the Equinox.

(4.) Date and time of the New Moon which occurs closest to the Equinox.

(5.) To find Nisan 1, add two hours and twenty-one minutes to account for the time difference at the meridian in Jerusalem.

Data for 2023

(1.) Date and time of the Vernal (Spring) Equinox: March 20 at 9:26 p.m.

(2.) Date and time of the New Moon preceding the Equinox: February 20 at 7:07 a.m.

(3.) Date and time of the New Moon following the Equinox: March 21 at 5:24 p.m.

(4.) Date and time of the New Moon which occurs closest to the Equinox: March 21 at 5:24 p.m.

(5.) Add two hours and twenty-one minutes to determine the beginning of Nisan 1 at the meridian in Jerusalem: March 21 at 7:45 p.m.

To determine the date of Nisan 1, let us remember that the Jewish day runs from 6:00 p.m. in the evening to 6:00 p.m. the following evening, unlike our customary midnight to midnight reckoning. By knowing the exact time in which Nisan 1 begins at Jerusalem in a given year, we are able to determine the date of Nisan 1 for that year.

For 2023, Nisan 1 begins on March 21 at 7:45 p.m. in Jerusalem. Backing up to the nearest preceding 6:00 p.m. indicates that Nisan 1 starts at 6:00 p.m. on March 21.

To determine the date of Nisan 14, the date to celebrate the Memorial, we need to count forward thirteen full days, which brings us to April 3. So, Nisan 14 starts at 6:00 p.m. on Monday, April 3, your local time.

Source: Astronomical Tables of the Sun, Moon, and Planets by Jean Meeus (Willmann-Bell, Inc.; 1983), pp. 3-43, 4-20.

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