Today is Easter Sunday for many Christians. For Orthodox Christians it is celebrated next Sunday. The reason for this difference stems in part from the day’s astronomical origin and the adoption of a calendar-based calculation of the date rather than astronomical observations.
Easter was originally tied to Passover, since according to the Bible, the crucifixion and resurrection of Christ occurred around Passover. Since the Hebrew calendar is based upon the cycles of the Moon, Hebrew months gradually shift relative to the solar year. For this reason “leap months” are added every few years to keep pace with the seasons. This is why Passover, and by extension Easter, occurs at a different day from year to year.
In 325 at the Council of Nicaea, the date of Easter was formalized as the Sunday after the full moon that follows the spring equinox (known as the Pascal Moon). But rather than using direct observations of the equinox and lunar cycle, the vernal equinox was defined as March 21. The calculation of the full moon was to be done using the Metonic moon cycle, which equates 235 synodic months to 19 solar years. In actuality, 19 years is 234.997 lunar months, so this is a reasonably good approximation. The advantage of using a calendar was that it ensured that Easter would be celebrated on the same day everywhere in the world, since the actual equinox and full moon could occur on different local days depending on your location.
The reason Orthodox Easter falls on a different day is due to a change in the solar calendar. The original solar calendar used in Europe was the Julian calendar, which had exactly 365 days in a year, with a leap year every four years. Since the actual solar year is 365.2421897 days, rather than exactly 365.25, the Julian calendar drifted relative to the seasons. So in 1582 the Catholic Church adopted the Gregorian calendar, which dropped the leap year for century years, and added the leap year back every 400 years. This made the official year 365.2425 days on average, which is more accurate. At that time the calculation of Easter shifted to the Gregorian March 21 equinox. The Orthodox Church continued to calculate Easter by the Julian calendar, which is why their Easter falls on a different Sunday.