Today is Pascha, or Easter in the Orthodox Church. The word Pascha is a transliteration of the Greek word Πάσχα), which in turn means "Pesach" or Passover. The day has been preceded by twelve weeks of preparation: the pre-lenten Sundays, Great Lent, and Holy Week; the day itself focuses upon the resurrection of Jesus Christ from the dead, which is the center of Christian faith. This site gives a summary of the day's religious services, and this site provides more information.
My wife Beth found this site of services happening in Jerusalem. http://www.huffingtonpost.com/2015/04/11/holy-fire-2015-orthodox-easter_n_7045904.html?ncid=tweetlnkushpmg00000067