The "Pink Moon" arrives on Good Friday, April 19, at 7:12 a.m. But it won't, itself, be pink. The name is due to one of the earliest widespread flowers of springtime: herb moss pink phlox, or “wild ground” phlox.
Other names for this moon have included the full Sprouting Grass Moon, the Egg Moon, and among coastal tribes, the full Fish Moon because this was the time that the shad swam upstream to spawn.
This full moon also dictates the date of Easter this year.
Easter, along with the many church holidays related to it — such as Ash Wednesday, Palm Sunday and Good Friday — are called "moveable feasts" because they do not fall on a fixed date on the Gregorian calendar, which follows the cycle of the sun and the seasons. Instead, these days follow a lunisolar calendar, similar to Jewish holidays.
According to a fourth century ruling, when the Christian holiday of Easter Sunday falls on the calendar is determined by the moon. The rule states that Easter is observed on the Sunday after the first full moon of spring, the so-called "Paschal Moon."
But wait, there was a full moon on March 20 — why wasn’t it the Paschal Moon?
Spring officially arrived in the Northern Hemisphere on Wednesday, March 20, at 5:58 p.m. Eastern time, with the arrival of the vernal equinox. Three hours and 45 minutes later, at 9:43 p.m. Eastern time, March's Worm Moon turned officially full. Because it occurred shortly after the equinox allows it to have the title of the first full moon of spring. So, according to the rule above, if the first full moon of spring occurred on Wednesday, March 20, it would stand to reason that Easter should have taken place the following Sunday, March 24, right?
Easter wasn't on March 24 because of two ecclesiastical rules that pertain to the Christian Church and its clergy:
• The Golden Number — While astronomers can tell us precisely to the exact minute when the moon will arrive opposite the sun that will brand it as a “full” moon, the Church follows its own methodologies in determining when the moon turns full. One important factor is something called the “Golden Number.” It is a rather arcane series of computations that in the end provides a date for Easter. Of course, on occasion, the date for the full moon does not exactly line up with the date that is provided by astronomy.
• Ecclesiastical Spring — This was the primary stumbling block for this year. From the ecclesiastical perspective, the first day of spring falls on March 21. But the date of spring differs in astronomy. So this year, since our March full moon fell on March 20 and not March 21, it was not recognized as the Paschal Moon by the Church. So, we must wait until the next full moon, on April 19. Finally, on Sunday, April 21, we will celebrate Easter Sunday. That’s within four days of the latest possible date for this moveable holiday.
Source: The Farmers' Almanac