Now the Feast of Unleavened Bread drew nigh, which is called the Passover. These words show that many of the readers for whom this Gospel was intended were foreigners, who were unacquainted with Jewish terms such as the "Passover."
Passover ( τὸ πάσχα, חסף) means, literally, "a passing." The feast so named commemorated the manner in which the chosen people were spared in Egypt when the destroying angel of the Lord passed over all Israelitish houses, which had been sprinkled with the blood of the lamb, without slaying the firstborn.
Dr. Farrar suggests that the Greek word πάσχω is a transliteration, with a sort of alliterative allusion to the Greek πάσχω, "I suffer." This greatest and most important of the Jewish feasts, which ever brought a great host of pilgrims to Jerusalem, was kept in the first month of the Jewish year (Nisan), from the 15th of the month, the day of full moon, to the 21st.
Roughly, this corresponded to the end of our March.