One more point: The Almanac is produced in Australia, but I tend to use the Northern Hemisphere seasons as that is where the majority of the Almaniacs live.

Calendar Systems (from Wikipedia)

Calendars in use on Earth are most frequently lunar, solar, lunisolar, planetary or arbitrary.

A lunar calendar is synchronised to the motion of the Moon (moon phases); an example is the Islamic calendar.

A solar calendar is based on perceived seasonal changes synchronised to the motion of the Sun; an example is the Persian calendar.

A lunisolar calendar is synchronised to the motions of both the Moon and the Sun; an example is the Jewish calendar.

A planetary calendar is a fixed period based on the number of visible moving objects in the sky; an example of this is the week.

An arbitrary calendar is not synchronised to either the Moon or the Sun; an example is the Julian day used by astronomers.

There are some calendars that appear to be synchronised to the motion of Venus, such as some of the ancient Egyptian calendars; synchronisation to Venus appears to occur primarily in civilisations near the Equator. 

When is Easter?

Easter is on a different date each year according to the Northern Vernal Equinox (may fall on March 20, 21 or 22) and the phases of the moon.  By knowing Easter's date we can determine many others in the Christian calendar, such as Shrove Tuesday (Pancake Day), Ash Wednesday and Lent, Mid-Lent, and more.

From Wikipedia:

“The timing of Easter depends on the Jewish Pesach, in English Passover, which commemorates the sparing of the Hebrew first-born, as recounted in Exodus, since it is during this holiday that Jesus is believed to have been resurrected.

The date of Easter

“Easter and the holidays that are related to it are moveable feasts, in that they do not fall on a fixed date in the Gregorian calendar (which follows the motion of the Sun and the seasons). Instead, they are based on a lunar calendar like that used by the Jews. At the First Council of Nicaea in 325 it was decided that Easter would be celebrated on the Sunday after the 14th day of the first lunar month of spring (in theory, the Sunday after the first full moon on or after the day of the vernal equinox). Eventually, all churches accepted the Alexandrian method of computing Easter, which set the northern hemisphere vernal equinox at 21 March (the actual equinox may fall one or two days earlier or later), and the date of the full moon was to be determined by using the Metonic cycle. A problem here is the difference between the western churches and the Eastern Orthodox Churches. The former now use the Gregorian calendar to calculate the date of Easter, while the latter still use the original Julian calendar. The World Council of Churches proposed a reform of the method of determining the date of Easter at a summit in Aleppo, Syria, in 1997. This reform would have eliminated the difference in the date between the Eastern and Western churches. The reform was due to be implemented starting in 2001, but it failed. See Reform of the date of Easter.

“Computing the date of Easter, known as computus, is somewhat complicated. The Wiki page explains the traditional tabular methods, but also has algorithms such as the one developed by the famous mathematician Carl Friedrich Gauss.

Source: Wikipedia

When is it this year? One explanation can be found in Chambers:

“Easter Day is always the first Sunday after the full moon which happens upon or next after the 21st day of March; and if the full moon happens upon a Sunday, Easter Day is the Sunday after.”
Chambers, R, (Ed.), The Book of Days: A miscellany of popular antiquities in connection with the calendar, etc, W & R Chambers, London, 1881 (1879 Edition is online; See The English Year: A Personal Selection from Chambers' Book of Days)

Or, probably better:

“Easter Sunday is the Sunday following the Paschal Full Moon (PFM) date for the year.   In June 325 A.D. astronomers approximated astronomical full moon dates for the Christian church, calling them Ecclesiastical Full Moon (EFM) dates. From 326 A.D. the PFM date has always been the EFM date after March 20 (which was the equinox date in 325 A.D.).” 
Source with some explanation, and discussion of popular errors

Lunabar will put moon phases, equinoxes, solstices, etc on your desktop

The date of Easter (US Naval Observatory)

The date of Easter (Anglican calculator)

Timing of the Crucifixion and the Resurrection (Christian Churches of God)