A solemn festival of the Jews, instituted in commemoration of their coming out of Egypt; because, the night before their departure, the destroying angel, who put to death the first-born of the Egyptians, passed over the houses of the Hebrews, without entering therein; because they were marked with the blood of the lamb, which was killed the evening before, and which for this reason was called the paschal lamb.
See Exodus 12:1-51 : Brown's Dict. article FEAST; and Mc'Ewen on the Types. p. 172.