Today (as I write, Friday, March 29, 2013), Christians worldwide are observing Good Friday — the Friday immediately prior to Easter Sunday.

Good Friday commemorates the crucifixion and death of Jesus Christ (traditionally believed to have occurred on a Friday), just as Easter Sunday commemorates the resurrection which Christians believe subsequently occurred (on the following Sunday).

On the calendar, Good Friday and Easter Sunday fall in close proximity each year to the Jewish holiday of Pesach or Passover. And, like Passover, these two closely connected Christian holidays do not fall upon the same dates each year. In other words, just because Good Friday happens to be on March 29 this year (2013) doesn’t mean that it’s always on March 29.

Last year, for instance, Good Friday was April 6, 2012; next year, by contrast, Good Friday will be April 18. It’s always the Friday before Easter Sunday, but Easter itself falls upon different dates in different years. Why? Put simply, because Easter is always the Sunday after the first full moon on or after March 21 (approximately the vernal or spring equinox in the northern hemisphere), which of course can and does vary each year.

This means that Easter Sunday (and Good Friday, which always precedes Easter by two days) may fall anywhere between late March and mid-to-late April.

Additionally, the matter of determining the date of Easter Sunday (and subsequently of Good Friday) each year is complicated further by the fact that different branches of Christianity utilize entirely different calendars in order to do so. Catholics and Protestants use the Gregorian (Western) calendar to identify March 21 and thereby subsequently determine Easter’s date; Eastern Orthodox churches, by contrast, rely instead upon the rather different Julian calendar, whose own March 21 usually falls on a different actual date than that of the Gregorian calendar’s March 21.

Depending upon the year, the Eastern Easter (and its own accompanying Good Friday) can be on the same dates as the Western Easter, or as much as over a month later.

The precise year in which the crucifixion and death of Jesus occurred is uncertain, but is estimated by many to have been circa 33 AD (or 33 CE).

The traditional Christian religious view is that Jesus Christ, understood by Christians to be the Son of God and a divine Savior, voluntarily died a self-sacrificial death in order to pay for, or atone for, all human sin. The New Testament gospel accounts paint a picture of Jerusalem’s Jewish leaders sentencing Jesus to death for the religious crime of blasphemy (for claiming to be divine).

Secular historians, by contrast, tend instead to view Jesus’s execution primarily as inflicted upon him by Jerusalem’s Roman overlords for the capital political crimes of treason and sedition. Death by crucifixion was Rome’s standard, and deliberately tortuous and humiliating, means of ridding itself of (and making examples of) troublesome political enemies and potential rabble-rousers — which is essentially how the Roman authorities viewed Jesus.

A traditionally somewhat somber holiday characterized by themes of mourning, Christians observe Good Friday in various ways, which may include fasting, repentance, reflection, prayer, and special church services.