Good Links by "Liliya829"
Summarization based on "calendars" - Lunar, Solar
Prior to approximately 325CE Passover and Easter were coincidental on the same Lunar calendar.
Summarized major reasons "Why" Pesach (Passover) is a month after Easter this year.
1. First Council of Nicaea (325 CE) keep Easter on a Sunday of Julian (Solar) calendar.
2. Pope Gregory XIII (1582 CE) reconstructed calendar to include "leap (Solar) year".
3. Pope Gregory revised "Easter tables":
--Easter falls on the first Sunday following the first "ecclesiastical" full moon that occurs on or after the day of the vernal equinox;
--this particular "ecclesiastical" full moon is the 14th day of a tabular lunation (new moon); and
--the vernal equinox is fixed as March 21.
4. Passover OT Commandment observance: Exodus 12:16 "In the first [month (Lunar)], on the fourteenth day of the month. . "
Note: Lunar month begins at first observable appearance of moon immediately following "new (dark) moon".
5. OT also Commandands: Deuteronomy 16:1: "Guard the month of spring, and make [then] the Passover offering."
6. "Month" (Lunar) is defined by OT Commandment: Exodus12:2 "This month shall be for you the beginning of months. . ."
7. Lunar calendar is approximately eleven days shorter than Solar calendar. The Sanhedrin (Jewish Eccliastical Supreme Court) constantly adjusted the calendar to ensure that Nissan (7th month), the month of the holiday of Passover, always fell during the spring season. This is accomplished through thirteen-month "leap years" which were added to the calendar approximately once every three years. During these years, a second month of Adar was added to the calendar.
No Sanhedrin stands today for approximately 1,950 years and the calendar has been astronomically fixed for 7 "leap years" on a 19 year cycle.