This is a somewhat complicated question, but one often asked, and so here is the answer in the simplest terms that still convey the gist of the issues.
Easter (or Pascha) is not a feast that is celebrated on a fixed day according to the Solar Calendar. The reason for this is that Easter is the Christian Celebration of Passover (“Pesach” in Hebrew, which is where the Greek word “Pascha” is derived from), and the Hebrew Calendar is a lunar calendar (based on the cycles of the moon) rather than a solar calendar (based on the annual cycle of the sun). However, while Passover can fall on any day of the week, the Christian Tradition is to celebrate Easter on the first Sunday after the first full moon after the vernal equinox (the day in spring when the night and the day is of equal length, and so after this day light is on the increase and darkness is on the decrease (in the northern hemisphere, that is).
The date of the vernal equinox is fixed at March 21st. The problem comes from the fact that the Julian Calendar, so named for Julius Caesar, who established it as the official calendar of the Roman Empire, has a slight error which took centuries to become apparent. The Julian calendar (which was the calendar used throughout the Christian world) assumes that the solar year is 365.25 days, and so normally a year is 365 days, but every fourth year (called the leap year) an extra day is added to the month of February, and the year is 366 days. The problem is that the actual solar year is 11 minutes shy of being precisely 365.25 days, which does not seem like much, but about every 128 years, this results in a day’s difference. As the centuries passed, the date that the vernal equinox actually occurred on began to slowly drift away from the date that it was supposed to occur. Pope Gregory XIII introduced a revised calendar in 1582, to address this problem, and to adjust the calendar such that the actual vernal equinox again occurred on March 21st. When Pope Gregory XIII introduced the Gregorian calendar, the difference between the two calendars was 10 days. Since that time, the difference has increased to 13 days. Unfortunately, however, five centuries prior to the calendar change, the Latin speaking western Church split away from the Orthodox Church, and so this calendar change was not accepted by any of the Orthodox Churches until the 1920’s. A few Churches accepted the change in the 20’s and several more followed suite in subsequent decades, but the Orthodox Churches of Russia, the Ukraine, Serbia, the Georgian Republic, the Holy Land, Jordan, Sinai, and Mount Athos, have continued to use the Julian Calendar to the present day. But when it comes to determining the date of Easter, even those Orthodox Churches which did accept the Gregorian Calendar continue to use the Julian Calendar.
Occasionally, both Easters coincide. More commonly, they are a week or so apart. But sometimes, as is the case this year, they are a month apart. But what should be noted is that the Jewish Passover continues to be celebrated by calculations that are similar to those of the Julian Calendar, and so Orthodox Easter is always after the Jewish Passover, and in close proximity to it. Western Easter often falls prior to the Jewish Passover, and as is the case this year, it can be as much as a month prior.
